## $hide=mobile$type=ticker$c=12$cols=3$l=0$sr=random$b=0 # ĐẶT MUA TẠP CHÍ / PURCHASE JOURNALS ### Issue 331 1. Can we find two positive integers$x, y$(written in decimal system) such that $$x+y=\underbrace{99 \ldots 9}_{n \text { times }}$$ and$y$is obtained by a permutation of the digits of$x$in the case where$n=2004$? and in the case where$n=2005$? 2. Prove that the number $$\sqrt{(a b-c d)(b c-d a)(c a-b d)}$$ is a rational, where$a, b, c$are rationals satisfying the condition$a+b+c+d=0$. 3. Find all integral solutions of the equation $$x^{2}+2003 x+2004 y^{2}+y=x y+2004 x y^{2}+2005.$$ 4. Prove that $$\frac{1}{1+a+b}+\frac{1}{1+b+c}+\frac{1}{1+c+a} \leq \frac{1}{2+a}+\frac{1}{2+b}+\frac{1}{2+c}$$ where$a, b, c$are positive real numbers satisfying the condition$a b c=1$. 5. Consider positive numbers$a, b, c$,$x, y, z$satisfying the conditions$a+b+c=4$and$a x+b y+c z=x y z$. Prove that $$x+y+z>4.$$ 6. Let be given a triangle$A B C$with its angled bisector$A M(M$lies on the side$B C)$. The line perpendicular to$B C$at$M$cuts the line$A B$at$N$. Prove that the angle$B A C$is right when and only when$M N=M C$. 7. The diagonals$A C$and$B D$of quadrilateral$A B C D$intersect at$K$so that$K A=K D$and$\widehat{A K D}=120^{\circ}$. From a point$M$on the side$B C$, draw$M N || A C$and$M Q || B D$($N$lies on$A B$,$Q$lies on$C D$). Find the locus of the circumcenters of triangles$M N Q$when$M$moves on the side$B C$. 8. Let be given two primes$p, q$satisfying$p>q>2$. Find all integers$k$so that the equation $$(p x-q y)^{2}=k x y z$$ has integral solution$(x, y, z)$satisfying$x y \neq 0$. 9. Consider the sequence of numbers$\left(u_{n}\right)(n=1,2,3, \ldots)$defined by $$u_{n}=n^{2^{n}},\,\forall n=1,2, \ldots$$ Put$\displaystyle x_{n}=\frac{1}{u_{1}}+\frac{1}{u_{2}}+\ldots+\frac{1}{u_{n}}$. Prove that the sequence$\left(x_{n}\right)$has a limit when$n$tends to infinity and the limit is an irrational. 10. Find all positive integers$n \geq 3$so that the following inequality occurs for$n$arbitrary real numbers$a_{1}, a_{2}, \ldots, a_{n}\left(a_{n+1}=a_{1}\right):$$$\sum_{1 \leq i<j \leq n}\left(a_{i}-a_{j .}\right)^{2} \leq\left(\sum_{i=1}^{n}\left|a_{i}-a_{i+1}\right|\right)^{2}$$ 11. Determine the form of triangle$A B C$knowing that its angles satisfy the condition $$\frac{\tan\frac{A}{2}}{1+\tan\frac{B}{2} \tan\frac{C}{2}}+\frac{\tan\frac{B}{2}}{1+\tan\frac{C}{2} \tan\frac{A}{2}}+\frac{\tan\frac{C}{2}}{1+\tan\frac{A}{2} \tan\frac{B}{2}}=\frac{1}{4 \tan\frac{A}{2} \tan\frac{B}{2} \tan\frac{C}{2}}.$$ 12. Consider the rectangular parallelepipeds$A B C D A_{1} B_{1} C_{1} D_{1}$such that the lengths of the side$A B=a$,$A D=b$,$A A_{1}=c$and the distance between the lines$A C$and$B C_{1}$are natural numbers. Find the least value of the volumes of these parallelepipeds. ### Issue 332 1. Find the remainder in the integer division of the number$S=a^{b}+b^{a}$by 5 , where$a=\overline{22 \ldots .2}$with 2002 digits$2, b=\overline{33 \ldots . .3}$with$2004$digits$3$(written in decimal system). 2. Let$A B C$be a triangle with$A B=A C$. On the perpendicular to$A C$at$C$, take a point$D$such that$B, D$lie on different sides of the line$A C$. Let$K$be the point of intersection of the line perpendicular to$A B$at$B$and the line passing through the midpoint$M$of$C D$, perpendicular to$A D .$Compare the measures of$K B$and$K D$. 3. Consider the equation$x^{2}-2 k x y^{2}+k\left(y^{3}-1\right)=0$where$k$is a positive integral parameter. Prove that this equation has integral solution$(x, y)$with$x>0$,$y>1$when and only when$k$is a perfect square. 4. Solve the equation $$\sqrt{x-\sqrt{x-\sqrt{x-\sqrt{x-5}}}}=5.$$ 5. Prove that $$\sqrt{a}+\sqrt[3]{a}+\sqrt[6]{a} \leq a+2$$ where$a$is a non negative real number. 6. Let$A B C$be a triangle with$\widehat{A} \geq \widehat{B} \geq \widehat{C}$and let$h_{\alpha}$,$h_{b}$,$h_{c}$be its altitudes issued respectively from$A$,$B$,$C$. Prove that $$\frac{h_{a}^{2}}{h_{b}^{2}}+\frac{h_{b}^{2}}{h_{c}^{2}}+\frac{h_{c}^{2}}{h_{a}^{2}} \geq \frac{h_{a}}{h_{b}}+\frac{h_{b}}{h_{c}}+\frac{h_{c}}{h_{a}} .$$ 7. Let$A B C D$be a parallelogram with$A B < B C$. The bisector of angle$B A D$cuts$B C$at$E$. The perpendicular bisectors of$B D$,$C E$intersect at$O$. The line passing through$C$, parallel to$B D$cuts the circle with center$O$and radius$O C$at$F$. Calculate the measure of angle$A F C$. 8. Prove that the polynomial $$P(x)=x^{4}-2003 x^{3}+(2004+a) x^{2}-2005 x+a$$ with integral parameter$a$has at most one integral root and has no multiple integral root (with multiplicity$>1$). 9. Prove that $$\left(x^{3}+3\right)\left(y^{3}+3\right)\left(z^{3}+3\right) \geq \frac{4}{27}(3 x y+3 y z+3 z x+x y z)^{2}$$ where$x, y, z$are real numbers. 10. Find all functions$f(x)$, defined on the interval$(0,+\infty)$, having derivative at$x=1$, and satisfying the condition $$f(x . y)=\sqrt{x} \cdot f(y)+\sqrt{y} \cdot f(x)$$ for all positive real numbers$x, y$. 11. Let$A_{1} A_{2} \ldots A_{n}$be a regular$n$-gone inscribed in a circle with radius 1 and let$M$be a point on the minor arc$\widehat{A_{1} A_{n}}$. Prove that a)$M A_{1}+M A_{3}+\ldots+M A_{n-2}+M A_{n}<\frac{n}{\sqrt{2}}$when$n$is odd, b)$M A_{1}+M A_{3}+\ldots+M A_{n-3}+M A_{n-1} \leq \frac{n}{\sqrt{2}}$when$n$is even. When does equality occur? 12. Let$O$be the centroid of a regular triangle$A B C$and$d$be the line orthogonal to the plane$(A B C)$at$O$. For every point$S$(distinct from$O$) on$d$, consider the pyramid$S A B C$. Let$\alpha$be the angle between a lateral face and the base, let$\beta$be the angle between two adjacent lateral faces of the pyramid. Prove that the quantity $$F(\alpha, \beta)=\tan^{2} \alpha\left(3 \tan^{2} \frac{\beta}{2}-1\right)$$ does not depend on$\alpha, \beta$when$S$moves on$d$. ### Issue 333 1. The fractions with positive numerators and denominators are arranged in the following order $$\frac{1}{1}, \frac{2}{1}, \frac{1}{2}, \frac{3}{1}, \frac{1}{3}, \frac{4}{1}, \frac{3}{2}, \frac{2}{3}, \frac{1}{4} \ldots, \frac{n}{1}, \frac{n-1}{2}, \ldots, \frac{n-k}{k+1}, \ldots,\frac{2}{n-1}, \frac{1}{n}, \ldots,$$ where there are no fractions of the form$\dfrac{m}{m}$,$m>1$. At which place in this sequence lies the fraction$\dfrac{2004}{2005} ?$2. Let$A B C$be a triangle with altitude$A H$. Let$M$,$N$be the orthogonal projections of$H$respectively on$A B$and$A C$. Prove that the condition$B M=C N$implies that$A B C$is an isosceles triangle with base$B C$. 3. Find all couple of positive integers$x$,$y$such that$\dfrac{x^{4}+2}{x^{2} y+1}$is a positive integer. 4. Solve the equation $$16 x^{4}+5=6\sqrt[3]{4 x^{3}+x}.$$ 5. Prove the inequality $$\frac{a+b}{a b+c^{2}}+\frac{b+c}{b c+a^{2}}+\frac{c+a}{c a+b^{2}} \leq \frac{1}{a}+\frac{1}{b}+\frac{1}{c}$$ where$a, b, c$are positive real numbers. 6. Let$A B C$be a triangle with orthocenter$H$(distinct from$A$,$B$,$C)$, and$M$be the midpoint of$B C$. The line passing through$H$perpendicular to$M H$cuts the line$A B$at$E$and cuts the line$A C$at$F$. Prove that$M E F$is an isosceles triangle with base$E F$. 7. From a point$M$in the interior of a rectangle$A B C D$, draw$A M$,$B M$then$C E \perp B M$at$E$,$D F \perp A M$at$F$. Let$N$be the point of intersection of$C E$and$D F$. Find the locus of the midpoint of$M N$when$M$moves in the interior of$A B C D$. 8. Prove that for every positive integer$n$, the difference $$s_{n}=\left(\sum_{k=1}^{n}\left[\frac{n}{k}\right]\right)-[\sqrt{n}]$$ is an even integer, where$[x]$denotes the integer part of$x$. 9. Solve the system of equations $$\begin{cases}x^{2}(x+1) &=2\left(y^{3}-x\right)+1 \\ y^{2}(y+1) &=2\left(z^{3}-y\right)+1 \\ z^{2}(z+1) &=2\left(x^{3}-z\right)+1\end{cases}$$ 10. Prove that $$\sum_{i=1}^{n} \sqrt{x_{i}^{2}+\frac{1}{x_{i}^{2}}} \geq\left(n+\frac{1}{n}\right) \cdot \sqrt{n^{2}+\frac{1}{n^{2}}} \cdot\left(\sum_{i=1}^{n} \frac{x_{i}}{1+x_{i}^{2}}\right)$$ where$x_{1}, x_{2}, \ldots, x_{n}$are$n$positive numbers satisfying the condition$x_{1}+x_{2}+\ldots+x_{n} \leq 1$. 11. Find the greatest value of the expression $$F=\sin A \cdot \sin ^{2} B \cdot \sin ^{3} C$$ where$A$,$B$,$C$are angles of a triangle. 12. Let$A B C D$be a tetrahedron inscribed in a sphere$(O)$with center$O$, let$G$be the centroid of$A B C D$, let$M$be a point lying in the interior of or on the sphere with diameter$O G$. The lines$M A$,$M B$,$M C$,$M D$cut again$(O)$respectively at$A_{1}$,$B_{1}$,$C_{1}$,$D_{1}$. Prove that $$V\left(A_{1} B_{1} C_{1} D_{1}\right) \geq V(A B C D)$$ where$V$denotes volume. ### Issue 334 1. Calculate the following sum$S$(consisting of$23$terms) $$S=\frac{1}{1.2 .3}+\frac{1}{2.3 .4}+\ldots+\frac{1}{(n-1) n(n+1)}+\ldots+\frac{1}{23.24 .25}$$ 2. Let$A B C$be a triangle with$\widehat{A} \neq 90^{\circ}, \hat{B} \neq 135^{\circ}$. Let$M$be the midpoint of$B C$. At the outside of$\triangle A B C$, construct the isosceles, right triangle$A B D$with base$A B$. The line passing through$A$perpendicular to$A B$and the line passing through$C$parallel to$M D$intersect at$E$. The line$A B$cuts$C E$at$P$and cuts$D M$at$Q$. Prove that$Q$is the midpoint of$B P$. 3. Find the least odd natural number$n$such that$n^{2}$is a sum of an odd number of perfect squares. 4. Find postitive numbers$a_{1}, a_{2}, a_{3}, a_{4}$satisfying the following conditions $$\frac{a_{1}^{2}}{a_{2}+a_{3}}+\frac{a_{2}^{2}}{a_{3}+a_{4}}+\frac{a_{3}^{2}}{a_{4}+a_{1}}+\frac{a_{4}^{2}}{a_{1}+a_{2}}=1$$ and $$a_{1}^{2}+a_{2}^{2}+a_{3}^{2}+a_{4}^{2} \geq 1.$$ 5. Find the least value of the expression $$T=\frac{a^{2}}{a^{2}+(b+c)^{2}}+\frac{b^{2}}{b^{2}+(c+a)^{2}}+\frac{c^{2}}{c^{2}+(a+b)^{2}},$$ where$a, b, c$are real numbers$(a b c \neq 0)$. 6. The incircle with center$I$of triangle$A B C$touches the sides$B C$,$C A$,$A B$respectively at$D$,$E$,$F$. The line passing through$A$perpendicular to$IA$cuts the lines$D E$,$D F$respectively at$M$,$N$. The line passing through$B$perpendicular to$IB$cuts the lines$E F$,$E D$respectively at$P$,$Q$. The line passing though$C$perpendicular to$I C$cuts the line$F D$,$F E$respectively at$S$,$T$. Prove that $$M N+P Q+S T \geq A B+B C+C A.$$ 7. Let be given an isosceles, right triangle$A B C$with base$B C$. Find the locus of points$M$satisfying the condition $$M B^{2}-M C^{2}=2 M A^{2}$$ 8. Let be given$n$distinct positive numbers$(n \geq 4)$. Prove that among them there are at least two numbers such that their sum and their difference do not coincide with any number of$n-2$other given nubers. 9. Prove that the sum$\displaystyle S_{n}=\sum_{k=0}^{n} \frac{1}{C_{n}^{k}}$has a finite limit when$n$tends to infinity and find this limit ($C_{n}^{k}$are binomial coefficients). 10. Find the least value of the sum $$P=\tan^{2} x \cdot \tan^{2} y+\tan^{2} y \cdot \tan^{2} z+\tan^{2} z \cdot \tan^{2} x$$ where$x, y, z$are positive numbers satisfying the conditions $$x+y+z=\frac{\pi}{2} ;\quad \cos (x-z) \leq \frac{7}{5} \sin y ;\quad \cos (x-y) \geq 3 \sin z.$$ 11. Let$d_{a}$,$d_{b}$,$d_{c}$be the lengths of the inner angle bisectors of triangle$A B C$issued respectively from the vertices$A$,$B$,$C$. Let$p$be the semi-perimeter of$\triangle A B C$. Prove that $$d_{d} \cdot \cos \frac{A}{2}+d_{b} \cdot \cos \frac{B}{2}+d_{c} \cos \frac{C}{2} \geq p(\cos A+\cos B+\cos C).$$ 12. Let$A B C$be a triangle right at$A$; let$M$be the midpoint of$B C$. On the line$d$passing through$M$perpendicular to the plane$(A B C)$, take a point$S$distinct from$M$. The plane$(Q)$containing$B C$, perpendicullar to the plane$(S A B)$, cuts the line$S A$at$D$. Determine the position of$S$on the line$d$so that the volume of the tetraheron$A B C D$attains its greatest value. ### Issue 335 1. Find the ratio of$A$and$B, where \begin{align}A= &\frac{1}{1.1981}+\frac{1}{2.1982}+\ldots+\frac{1}{n(1980+n)}+\ldots+\frac{1}{25.2005},\\ B= & \frac{1}{1.26}+\frac{1}{2.27}+\ldots+\frac{1}{m(25+m)}+. .+\frac{1}{1980.2005}.\end{align} (A$consists of$25$terms,$B$consists of$1980$terms.) 2. Let$A B C$be an isosceles, right triangle with base$B C$. Let$M$and$N$be respectively the midpoints of$A B$and$A C$. Draw$N H$perpendicular to$CM$at$H$, draw$H E$perpendicular to$A B$at$E$. Prove that the triangle$A B H$is isosceles and the line$H M$is the bisector of angle$B H E$. 3. Find integral solutions of the equation $$2\left(x^{2}-y^{2}\right)^{2}=x^{2}+y^{2}+2 z^{2}.$$ 4. Solve the system of equations $$\begin{cases} x+y+z &=1 \\ \dfrac{x}{y}+\dfrac{y}{z}+\dfrac{z}{x} &= \dfrac{x+y}{y+z}+\dfrac{y+z}{x+y}+1\end{cases}$$ where$x, y, z$are positive numbers. 5. Find the least value of the expression $$P=\left(3+\frac{1}{a}+\frac{1}{b}\right)\left(3+\frac{1}{b}+\frac{1}{c}\right)\left(3+\frac{1}{c}+\frac{1}{a}\right)$$ where the positive numbers$a, b, c$satisfy the condition$a+b+c \leq \dfrac{3}{2}$. 6. Let$A B C D$be a parallelogram with obtuse angle$B A D$. In the interior of angle$B A D$, construct the isosceles right triangle$A D E$with base$A E$and the isosceles right triangle$A B F$with base$A F$. Let$M$be the midpoint of$E F$. The segment$M B$cuts$C F$at$K$, the segment$M D$cuts$C E$at$H$. Prove that$H K$is parallel to$B D$. 7. Let$A B C$be an isosceles triangle with$\widehat{A B C}=120^{\circ}$and let$D$be the point of intersection of the line$B C$with the tangent at$A$of the circumcircle of triangle$A B C$. The line passing through$D$and the circumcenter$O$cuts the lines$A B$and$A C$respectively at$E$and$F$. Let$M$and$N$be respectively the midpoints of$A B$and$A C$. Prove that the lines$A O$,$M F$,$N E$are concurrent. 8. Consider the polynomial $$T(x)=x^{3}+17 x^{2}-1239 x+2001 .$$ Put$T_{1}(x)=T(x), T_{n+1}(x)=T\left(T_{n}(x)\right)$for every$n=1,2,3, \ldots$Prove that there exists an integer$n>1$such that$T_{n}(x)-x$is divisible by 2003 for every integer$x$. 9. Consider the sequence of numbers$\left(x_{n}\right)(n=1,2,3, \ldots)$defined by $$x_{1}=2,\quad x_{n+1}=\frac{1}{2}\left(x_{n}^{2}+1\right),\,\forall n=1,2,3, \ldots$$ Put$\displaystyle S_{n}=\frac{1}{1+x_{1}}+\frac{1}{1+x_{2}}+\ldots+\frac{1}{1+x_{n}}$. Find the integral part of$S_{2005}$and find the limit of$S_{n}$when$n$tends to infinity. 10. Consider the sequence of numbers$\left(a_{n}\right)(n=1,2,3, \ldots)$defined by $$a_{1}=\frac{1}{2},\quad a_{n+1}=\left(\frac{1-\left(1-a_{n}^{2}\right)^{1 / 2}}{2}\right)^{1 / 2},\,\forall n=1,2,3, \ldots$$ Prove that$a_{1}+a_{2}+\ldots+a_{2005}<1,03$. 11. Prove that for every triangle$A B C$, we have $$\cos A+\cos B+\cos C \leq 1+\frac{1}{6}\left(\cos ^{2} \frac{A-B}{2}+\cos ^{2} \frac{B-C}{2}+\cos ^{2} \frac{C-A}{2}\right).$$ 12. In a triangle$A B C$, let$B C=a$,$C A=b$,$A B=c$and let$S$be its area. Let the points$M$,$N$,$P$lie respectively on the sides$B C$,$C A$,$A B$. Prove that $$a b \cdot M N^{2}+b c \cdot N P^{2}+c a \cdot P M^{2} \geq 4 S^{2}$$ when does equality occur? ### Issue 336 1. Compare the following fractions (not by direct calculations) $$\frac{222221}{222222} ; \frac{444443}{444445} ; \frac{666664}{666667} ; \frac{888885}{888889}$$ 2. Let$A B C$be a triangle with$\widehat{A C B}=45^{\circ}$and obtuse angle$A$. Draw the ray$B D$cutting the opposite ray of$C A$at$D$so that$\widehat{C B D}=\widehat{A B C}$. Draw$A H$perpendicular to$B D$at$H .$Calculate$\widehat{C H D}$. 3. If the lengths of the sides of a right triangle are integers, can its area be a perfect square? 4. Solve the following system of equations, where$a, b, c$are given positive numbers $$\begin{cases}\dfrac{a}{x}-\dfrac{b}{z} &=c-z x \\ \dfrac{b}{y}-\dfrac{c}{x} &=a-x y \\ \dfrac{c}{z}-\dfrac{a}{y} &=b-y z\end{cases}$$ 5. Prove the inequality $$\frac{1}{a}+\frac{1}{b}-\left(\sqrt{\frac{a}{b}}-\sqrt{\frac{b}{a}}\right)^{2} \geq 2 \sqrt{2}$$ where$a, b$are real positive numbers satisfying$a^{2}+b^{2}=1$. 6. Let$A B C$be an acute triangle with orthocenter$H$. Prove that $$\frac{H A}{B C}+\frac{H B}{C A}+\frac{H C}{A B} \geq \sqrt{3}.$$ When does equality occur? 7. Let$M$be a point in the interior of a triangle$A B C$with$B C=a$,$C A=b$,$A B=c$. Let$h_{a}$,$h_{b}$,$h_{c}$be respectively the distances from$M$to the lines$B C$,$C A$,$A B$. Determine the position of$M$so that the product$h_{a} \cdot h_{b} \cdot h_{c}$attains its greatest value and calculate this value. 8. Let be given an odd prime$p$and the polynomial$Q(x)=(p-1) x^{p}-x-1 .$Prove that there exists an infinite number of positive integers a such that$Q(a)$is divisible by$p^{p}$. 9. Solve the equation $$x^{4}+4 a x^{3}+6 b^{2} x^{2}+4 c^{3} x+1=0$$ where$a, b, c$are positive real numbers,$a \leq 1$, knowing that it has four real roots. 10. Calculate the sum of$2 n$terms $$S=\frac{1}{2} C_{2 n}^{1}-\frac{1}{3} C_{2 n}^{2}+\ldots+(-1)^{k} \frac{1}{k} \cdot C_{2 n}^{k-1}+\ldots+(-1)^{2 n+1} \frac{1}{2 n+1} C_{2 n}^{2 n}$$ where$C_{n}^{k}$are binomial coefficients. 11. Prove that for every triangle$A B C$, we have a)$\cos A+\cos B+\cos C+\cot A+\cot B+\cot C \geq \frac{3}{2}+\sqrt{3}$. b)$\sqrt{3}(\cos A+\cos B+\cos C)+\cot\frac{A}{2}+\cot\frac{B}{2}+\cot\frac{C}{2} \geq \frac{9 \sqrt{3}}{2}$. 12. Let be given a tetrahedron$A B C D$. Take a point$M$in the interior of triangle$A B C$and the points$A^{\prime}$,$B^{\prime}$,$C^{\prime}$lying on$D A$,$D B$,$D C$respectively so that$M A^{\prime}$,$M B^{\prime}$,$M C^{\prime}$are parallel respectively to the planes$(D B C)$,$(D C A)$,$(D A B)$. Prove that the circumsphere of tetrahedron$A^{\prime} B^{\prime} C^{\prime} D$passes through a fixed point distinct from$D$when$M$moves in the interior of triangle$A B C$. ### Issue 337 1. Find the first four digits (on the left) of the number$S$which is the following sum of 1000 terms $$S=1+2^{2}+3^{3}+\ldots+n^{n}+\ldots+1000^{1000}.$$ 2. Let$A B C$be a triangle with$A B>A C$. Take the points$M, N$respectively on the sides$A B$and$A C$such that$A M=A N$. Let$K$be the point of intersection of$B N$and$C M$. Compare the lengths of$K B$and$K C$. 3. Consider a triangle such that the measures of its sides are three consecutive integers greater than$3$and its area is also an integer. Prove that the triangle has an altitude which divides it into two small triangles such that the measures of the sides of both small triangles are integers. 4. Solve the equation $$x^{3}-3 x^{2}+2 \sqrt{(x+2)^{3}}-6 x=0$$ 5. Find the greatest value of the expression$T=2 a c+b d+c d$, where$a, b, c, d$are real numbers satisfying the conditions $$4 a^{2}+b^{2}=2,\quad c+d=4.$$ 6. Let$A B C$be a triangle. Its angle bisectors$B M$,$C N$($M$on the side$A C$,$N$on the side$A B$) intersect at$D$. Prove that$\triangle A B C$is right at$A$when and only when $$2 B D \cdot C D=B M \cdot C N.$$ 7. Let be given an angle$\widehat{x P y}=30^{\circ}$.$A$is an arbitrary point on the ray$P x$,$B$is an arbitrary point on the ray$P y$such that$A B=d$($d$is a given constant). Find the greatest value of the perimeter and the greatest value of the area of triangle$P A B$. 8. Let$f: \mathbb Z \rightarrow \mathbb Z$be a function satisfying the conditions $$f(0)=1,\quad f(f(x))=x+4 f(x),\,\forall x \in \mathbb Z.$$ Find all natural numbers$n(n \geq 1)$such that$f_{n}(0)$is divisible by$20^{11^{2005}}$, where$f_{1}(x)=f(x)$,$f_{n}(x)=f\left(f_{n-l}(x)\right)$for all$n \geq 2$. 9. Find the greatest value of the expression $$P=(x-y)(y-z)(z-x)(x+y+z)$$ where$x, y, z$are real numbers belong to the segment$[0 ; 1]$. 10. Let be given a natural number$n \geq 2$and positive real numbers$a, b$with$a<b$. Find the greatest value of the expression $$Q=\sum_{1 \leq i<j \leq n}\left(x_{i}-x_{j}\right)^{2},$$ where$x_{1}, x_{2}, \ldots, x_{n}$are$n$real numbers belong to the segment$[a, b]$. 11. A line passing through the incenter of triangle$A B C$cuts the sides$A B$and$A C$respectively at$M$and$N$. Prove that $$\frac{B M \cdot C N}{A M \cdot A N} \leq \frac{B C^{2}}{4 A B \cdot A C}$$ 12. Let be given a tetrahedon$A B C D$. Let$A_{1}$,$B_{1}$,$C_{1}$,$D_{1}$be respectively the centroids of the faces opposite to the vertices$A$,$B$,$C$,$D$. The lines$A A_{1}$,$B B_{1}$,$C C_{1}$,$D D_{1}$cut the circumsphere of$A B C D$again at$A_{2}$,$B_{2}$,$C_{2}$,$D_{2}$respectively. Prove that $$\frac{A A_{1}}{A A_{2}}+\frac{B B_{1}}{B B_{2}}+\frac{C C_{1}}{C C_{2}}+\frac{D D_{1}}{D D_{2}} \leq \frac{8}{3}.$$ ### Issue 338 1. Can the expression$x^{4}+y^{4}+z^{4}$take the value 2004 for positive fractions$x, y, z$? 2. Let be given a triangle$A B C$. Take the point$D$on the half-plane with boundary$A B$not containing$C$such that$D A \perp A B$and$A D=A B$. Take the point$E$on the half-plane with boundary$A C$not containing$B$such that$E A \perp A C$and$A E=A C$. Compare the areas of the triangles$A D E$and$A B C$. 3. Find all integral solutions of the equation $$(2 x-y-2)^{2}=7\left(x-2 y-y^{2}-1\right).$$ 4. Solve the equation $$\sqrt{5 x-1}+\sqrt[3]{9-x}=2 x^{2}+3 x-1.$$ 5. Prove the inequality $$\frac{a}{p b+q c}+\frac{b}{p c+q d}+\frac{c}{p d+q a}+\frac{d}{p a+q b} \geq \frac{4}{p+q}$$ for positive numbers$a, b, c, d, p, q$satisfying$p \geq q$. Does the inequality hold for$p<q$? 6. In plane let be given two lines$d_{1}$,$d_{2}$intersecting at$K$and let$M$be a point not lying on$d_{1}$,$d_{2}$. A line$d$passing through$M$cuts$d_{1}$and$d_{2}$respectively at$A$and$B$(distinct from$K$). Draw$A P \perp d_{2}$at$P$,$B Q \perp d_{1}$at$Q$. Prove that the line$P Q$passes through a fixed point when the line$d$turns around$M$. 7. Let$A B C$be a triangle right at$C$, let$C D$be its altitude and let$S$be its area. Let$(O)$be the circle with diameter$A B$, let$\left(O_{1}\right)$,$\left(O_{2}\right)$be the circles touching$(O)$, touching the segment$C D$and touching the segment$A B$respectively at$E$and$F$. Prove that $$S=\frac{A D \cdot B D \cdot A E \cdot B F}{2 E D \cdot F D}$$ 8. Find the least positive integer$n$such that there exists a polynomial of degree$n$with integral coefficients$P(x)$satisfying the following conditions •$P(0)=1$,$P(1)=1$, • for every positive integer$m$, the remainder of the division of$P(m)$by$2003$is$0$or$1$. 9. Find all functions$f: R \rightarrow R$satisfying the condition $$f\left(x^{2}+f(y)\right)=y+x f(x)$$ for all real numbers$x, y$. 10. Let$\left(F_{n}\right)(n=1,2, \ldots)$be the Fibonacci sequence $$F_{1}=F_{2}=1,\quad F_{n+1}=F_{n}+F_{n-1},\,\forall n=2,3,4,\ldots$$ Prove that if$a \neq-\frac{F_{n+1}}{F_{n}}$for every$n=1,2,3$, then the sequence of numbers$\left(x_{n}\right)$, where $$x_{1}=a,\quad x_{n+1}=\frac{1}{1+x_{n}},\,\forall n=1,2,3, \ldots$$ is defined and it has a finite limit when$n$tends to infinity and find this limit. 11. Let$A B C$be a triangle with$B C=a$,$C A=b, A C=b$and with area$S$. Let$m_{a}$,$m_{b}$,$m_{c}$be respectively the lengths of the medians issued from$A$,$B$,$C$. Prove that $$S \leq \frac{a^{2} m_{a}^{2}+b^{2} m_{b}^{2}+c^{2} m_{c}^{2}}{\sqrt{3}\left(a^{2}+b^{2}+c^{2}\right)}$$ When does equality occur? 12. Let$R$and$r$be respectively the radii of the circumsphere and the inscribed sphere of a tetrahedron$A B C D$with$A B=C D$,$A C=B D$,$B C=A D$. Prove the inequality $$\frac{\sin A+\sin B+\sin C}{\sqrt{\cos A \cdot \cos B \cdot \cos C}}>\frac{3 R}{2 r}$$ where$A$,$B$,$C$are the angles of triangle$A B C$. ### Issue 339 1. How many digits has the number$5^{50}$(written in decimal system)? 2. Find the least value of the fractions of the form$\dfrac{a b}{a c+b d}$, where$a, b, c, d$are positive integers satisfying the condition$a+b=c+d=2006$. 3. Has the equation $$x^{2005}+y^{2005}=2007^{2005}$$ integral solutions? 4. Solve the equation $$2 \sqrt[4]{27 x^{2}+24 x+\frac{28}{3}}=1+\sqrt{\frac{27}{2} x+6}$$ 5. Find the least value of the expression $$P=\frac{1}{1+x_{1} x_{2}}+\frac{1}{1+x_{2} x_{3}}+\frac{1}{1+x_{3} x_{4}}+\frac{1}{1+x_{4} x_{5}}+\frac{1}{1+x_{5} x_{1}}$$ where$x_{1}, x_{2}, x_{3}, x_{4}, x_{5}$are positive real numbers satisfying the condition $$x_{1}^{2}+x_{2}^{2}+x_{3}^{2}+x_{4}^{2}+x_{5}^{2}=2005.$$ 6. Let be given a square$A B C D$. The line perpendicular to$A C$at$C$cuts the lines$A B$,$A D$respectively at$E$,$F$. Prove that $$B E \cdot \sqrt{C F}+D F \cdot \sqrt{C E}=A C \cdot \sqrt{E F}.$$ 7. Let$I$be the incenter of triangle$A B C$and let$m_{a}$,$m_{b}$,$m_{c}$be the measures of the medians of$A B C$issued respectively from$A$,$B$,$C$. Prove that $$\frac{I A^{2}}{m_{a}^{2}}+\frac{I B^{2}}{m_{b}^{2}}+\frac{I C^{2}}{m_{c}^{2}} \leq \frac{4}{3} .$$ 8. Let be given two positive real numbers$u$,$v$. Consider the expression $$P=x^{2}+u y^{2}+v z^{2},$$ where$x, y, z$are arbitrary real positive numbers satisfying the condition$x y+y z+z x=1$. Prove that the least value of$P$equals$2 t$, where$t$is the root lying in the interval$(0 ; \sqrt{u v})$of the equation $$2 x^{3}+(u+v+1) x^{2}-u v=0.$$ Find prime numbers$u, v$so that$2 t$is a rational number. 9. Consider the sequence of numbers$\left(x_{n}\right)(n=1,2,3, \ldots)$defined by$x_{n}=a_{n}^{a_{n}}$, where $$a_{n}=\frac{(2 n) !}{(n !)^{2} \cdot 2^{2 n}},\,\forall n=1,2,3, \ldots$$ Prove that the sequence$\left(x_{n}\right)$has a limit when$n$tends to infinity and find this limit. 10. Let$a$be a real number belonging to the interval$(0 ; 1)$. Find all functions$f:\mathbb R \rightarrow \mathbb R$, continuous at$x=0$, satisfying the condition $$f(x)-2 f(a x)+f\left(a^{2} x\right)=x^{2}$$ fore every$x \in \mathbb R$. 11. In plane, let be given a cirle with$O P=d>0$. Two arbitrary chords$A B$,$C D$passing through$P$, form an angle with constant measure$\alpha\left(0^{\circ}<\alpha \leq 90^{\circ}\right)$. Find the greatest value and the least value of the sum$A B+C D$, when the chords$A B$,$C D$vary and determine the positions of$A B$,$C D$in these cases. 12. Let$P A B C$be a tetrahedron such that$P A$,$P B$,$P C$are perpendicular each to the others. Let$S=S_{A B C}$,$S_{1}=S_{P A B}$,$S_{2}=S_{P B C}$,$S_{3}=S_{P A C}$. Prove that $$\frac{S_{1}^{2}}{S^{2}+S_{1}^{2}}+\frac{S_{2}^{2}}{S^{2}+S_{2}^{2}}+\frac{S_{3}^{2}}{S^{2}+S_{3}^{2}} \leq \frac{3}{4} .$$ ### Issue 340 1. Let$x$be the sum of the digits of the number$a=3^{2004}+2005$, let$y$be the sum of the digits of the number$x$and let$z$be the sum of the digits of the number$y$. Find$z$. 2. Find the least value of the expression $$A=|7 x-5 y|+|2 z-3 x|+|x y+y z+z x-2000|+t^{2}-t+2005,$$ where$x, y, z, t$are rational numbers. 3. Find the least value of the expression$A=x^{2}+y^{2}$, where$x, y$are positive integers and$A$is divisible by$2004$. 4. Solve the equation $$13 \sqrt{x-1}+9 \sqrt{x+1}=16 x.$$ 5. Find the least value and the greatest value of the expression $$P=\frac{x+y}{1+z}+\frac{y+z}{1+x}+\frac{z+x}{1+y}$$ where$x, y, z$are real numbers belonging to the segment$\left[\frac{1}{2} ; 1\right]$. 6. Let$A B C$be a triangle. For a point$M$inside the triangle, let$E$be the point of intersection of$A M$and$B C$, let$F$be the point of intersection of$C M$and$A B$. Let$N$be the reflection of$B$in the midpoint of$E F$. Prove that the line$M N$passes through a fixed point when$M$move inside triangle$A B C$. 7. Let$S$be the area of triangle$A B C$with$B C=a$,$C A=b$,$A B=c$. Prove that $$S \leq \frac{\sqrt{3}}{4} \cdot \sqrt[3]{a^{2} b^{2} c^{2}}.$$ When does equality occur? 8. Let$f(x)$be a polynomial of degree$3$with integral coefficients and leading coefficient$1$. Suppose that$f(0)+f(1)+f(-1)$is not divisible by$3$. Find$\displaystyle\lim_{n\to\infty} \sqrt[3]{f(n)}$when the integer$n$tends to infinity. 9. Does there exist a function$f:(0 ;+\infty) \rightarrow(0 ;+\infty)$satisfying the condition $$f^{2}(x) \geq f(x+y)(f(x)+y)$$ for all positive real numbers$x, y$? 10. Prove that $$\sum_{i=1}^{n} \sum_{j=1}^{n} \frac{a_{i} a_{j}}{C_{k+i+j}^{k+2}} \geq 0$$ where$n, k$are non negative integers,$n>1$,$a_{1}, a_{2}, \ldots, a_{n}$are$n$arbitrary real numbers, and$C_{m}^{r}=\dfrac{m !}{r !(m-r) !}$11. Let$I$be the incenter of a triangle$A B C$with$B C=a$,$C A=b$,$A B=c$. Put$I A=d_{a}$,$I B=d_{b}$,$I C=d_{c}$. Prove that $$\sqrt{a\left(b c-d_{a}^{2}\right)}+\sqrt{b\left(c a-d_{b}^{2}\right)}+\sqrt{c\left(a b-d_{c}^{2}\right)} \leq \sqrt{6 a b c}$$ 12. Let$P$be a plane turning around the centroid of a regular tetrahedron$A_{1} A_{2} A_{3} A_{4}$with side$c$. Let$B_{i}$be the projection of$A_{i}(i=1,2,3,4)$on the plane$P .$Find the greatest value of the sum $$T=A_{1} B_{1}^{4}+A_{2} B_{2}^{4}+A_{3} B_{3}^{4}+A_{4} B_{4}^{4}$$ in term of$c$and determine the position of$P$when the sum attains its greatest value. ### Issue 341 1. Find the last decimal digit of the following sum of$502$terms $$S=2^{1}+3^{5}+4^{9}+\ldots+n^{4 n-7}+\ldots+503^{2005}.$$ 2. Let$A B C$be an isosceles triangle and$D$be the point on its base$B C$such that$C D=2 B D$. Compare the measures of the angles$\widehat{B A D}$and$\dfrac{1}{2} \widehat{C A D}$. 3. Find all positive integral solutions of the equation $$x^{y}+x^{z}+x^{t}=x^{2005}.$$ 4. Solve the equation $$\left(x^{2}-12 x-64\right)\left(x^{2}+30 x+125\right)+8000=0 .$$ 5. Find the least value of the sum $$S=\frac{x y}{z}+\frac{y z}{x}+\frac{z x}{y}$$ where$x, y, z$are positive real numbers satisfying the condition$x^{2}+y^{2}+z^{2}=1$. 6. Let$A B C$be an equilateral triangle and$D$be the reflection of$B$in the line$A C$. A line passing through$B$cuts the lines$A D$,$C D$respectively at$M$,$N$. The lines$A N$and$C M$intersect at$E$. Prove that the points$A$,$C$,$D$,$E$are concyclic. 7. Let$A B C$be an equilateral triangle and$D$be the reflection of$B$in the line$A C$, and$M$be the point on the ray$B C$such that$B M=\dfrac{4}{3} B C$. The line$A M$cuts$C D$at$N$. Take a point$E$on the segment$A B$and a point$F$on the segment$A D$so that the lines$C E$,$N F$are parallel. Calculate the measure of the angle$E O F$, where$O$is the midpoint of$A C$. 8. Prove that for every positive integer$n>2$, there exist$n$distinct positive integers such that the sum of these numbers is equal to their least common multiple and is equal to$n !$. T9/341. Prove that $$2 x^{2}+y^{2}+5 z^{2}+6 x y+7 x z+2 y z>0$$ for real numbers$x, y, z$satisfying the conditions$x+y+z<0$and$4 x z>y^{2}$. 9. Find the least value of the expression $$P=\sqrt{(x-a)^{2}+(y-b)^{2}}+\sqrt{(x-c)^{2}+(y-d)^{2}}$$ where$a, b, c, d, x, y$are real numbers satisfying the following conditions $$\begin{cases}a^{2}+b^{2}+40 &=8 a+10 b,\\ c^{2}+d^{2}+12& =4 c+6 d,\\ 3 x &=2 y+13 .\end{cases}$$ 10. Consider the convex quadrilaterals$A B C D$having inscribed circle. Let$M$,$N$,$P$,$Q$be the touching points of the inscribed circle with the sides$A B$,$B C$,$C D$,$D A$respectively. Find the least value of the expression $$T=\frac{A M^{2}}{x_{1} x_{2}}+\frac{B N^{2}}{x_{2} x_{3}}+\frac{C P^{2}}{x_{3} x_{4}}+\frac{D Q^{2}}{x_{4} x_{1}}$$ where$\left\{x_{1}, x_{2}, x_{3}, x_{4}\right\}$is a permutation of the measure$a=A B$,$b=B C$,$c=C D$,$d=D A$. 11. Let$O A B C$be a tetrahedron such that the sides$O A$,$O B$,$O C$are orthogonal each to others. Let$H$be the orthocenter of triangle$A B C$. The line$A H$cuts$B C$at$K$. The line passing through the incenters of the triangles$O B K$,$O C K$cuts$O B$,$O C$at$M$,$N$respectively. The plane bisecting the dihedral angle$[B,O A,H]$cuts$B C$at$D$, the plane bisecting the dihedral angle$[C, O A, H]$cuts$B C$at$E$. Prove the inequality for volumes $$V_{O A D E} \cdot V_{O A M N} \leq \frac{\sqrt{2}-1}{2} V_{O A B C}^{2}$$ ### Issue 342 1. Find all whole numbers $$A=2005^{n}+n^{2005}+2005 n$$ is divisible by$3$. 2. Let$A B C$be a triangle with$\widehat{A B C}=\widehat{A C B}=36^{\circ}$. On the ray bisecting the angle$A B C$take the point$N$so that$\widehat{B C N}=12^{\circ}$. Compare the measures of$C N$and$C A$. 3. Find all integral solutions of the equation $$\left(x^{2}+y^{2}+1\right)^{2}-5 x^{2}-4 y^{2}-5=0 .$$ 4. Find the value of the expression $$P=a^{2005}+b^{2005}+c^{2005}$$ where$a, b, c$are real numbers, distinct from$O$, satisfying the following conditions 5. Prove the following inequality for nonnegative real numbers $$\frac{2 \sqrt{2}}{\sqrt{x+1}}+\sqrt{x} \leq \sqrt{x+9}.$$ When does equality occur? 6. Let$A B C$be a triangle with$A B=A C$,$\widehat{B A C}=80^{\circ}$. Take the point$M$inside the triangle so that$\widehat{M A C}=20^{\circ}$,$\widehat{M C A}=30^{\circ}$. Find the measure of$\widehat{M B C}$. 7. Let be given a circle$(O)$with center$O$, two chords$C A$,$C B$not passing through$O$,$B A \neq B C$. The line passing through$A$, perpendicular to the line$O B$, cuts the line$C B$at$N$. Let$M$be the midpoint of$A N$. The line$B M$cuts the circle$(O)$at$B$and$D$. Let$E$be the point such that$O E$is a diameter of the circle passing through$B$,$D$,$O$. Prove that the points$A$,$C$,$E$are collinear. 8. Let$M$be a set consisting of$2005$positive numbers$a_1,\ldots,a_{2005}$. Consider all positive numbers non empty subsets$T_{i}$of$M$and let$s_{i}$be the sum of the numbers belonging to$T_{i}$. Prove that the set of numbers$s_{i}$can be partitioned into$2005$non empty disjoint subsets so that the ratio of two arbitrary numbers belonging to a such subset does not exceed$2$. 9. The sequence of numbers$\left(x_{n}\right)(n=1,2, \ldots)$is defined by $$x_{1}=1, \quad x_{n+1}=\sqrt{x_{n}\left(x_{n}+1\right)\left(x_{n}+2\right)\left(x_{n}+3\right)+1},\,\forall n=1,2, \ldots.$$ Put$\displaystyle y_{n}=\sum_{i=1}^{n} \frac{1}{x_{i}+2}(n=1,2, \ldots)$. Find$\displaystyle\lim_{n \rightarrow \infty} y_{n}$. 10. Prove the inequality $$\left(a^{2}+\frac{1}{a b}\right)^{\alpha}+\left(b^{2}+\frac{1}{b c}\right)^{\alpha}+\left(c^{2}+\frac{1}{c a}\right)^{\alpha} \geq 3.2^{\alpha}$$ where$a, b, c$are positive numbers and$\alpha$is a rational number greater than$1$. When does equality occur? 11. Let$r$and$R$be respectively the inradius and the circumradius of a triangle$A B C$. Prove that $$\cos A \cdot \cos B \cdot \cos C \leq \frac{r^{2}}{2 R^{2}}.$$ When does equality occur? 12. In space, let be given a sphere$(S)$and a line$\Delta$not intersecting$(S)$. For each point$M$on$\Delta$, take three arbitrary tangent planes to$(S)$, passing through$M$and touching$(S)$at$A$,$B$,$C$. Prove that the planes$A B C$contain a fixed line when$M$moves on$\Delta$. ##$hide=mobile$type=ticker$c=36$cols=2$l=0$sr=random$b=0

Name

Abel,5,Albania,2,AMM,2,Amsterdam,4,An Giang,45,Andrew Wiles,1,Anh,2,APMO,21,Austria (Áo),1,Ba Lan,1,Bà Rịa Vũng Tàu,77,Bắc Bộ,2,Bắc Giang,62,Bắc Kạn,4,Bạc Liêu,18,Bắc Ninh,53,Bắc Trung Bộ,3,Bài Toán Hay,5,Balkan,41,Baltic Way,32,BAMO,1,Bất Đẳng Thức,69,Bến Tre,72,Benelux,16,Bình Định,65,Bình Dương,38,Bình Phước,52,Bình Thuận,42,Birch,1,BMO,41,Booklet,12,Bosnia Herzegovina,3,BoxMath,3,Brazil,2,British,16,Bùi Đắc Hiên,1,Bùi Thị Thiện Mỹ,1,Bùi Văn Tuyên,1,Bùi Xuân Diệu,1,Bulgaria,6,Buôn Ma Thuột,2,BxMO,15,Cà Mau,22,Cần Thơ,27,Canada,40,Cao Bằng,12,Cao Quang Minh,1,Câu Chuyện Toán Học,43,Caucasus,3,CGMO,11,China - Trung Quốc,25,Chọn Đội Tuyển,515,Chu Tuấn Anh,1,Chuyên Đề,125,Chuyên SPHCM,7,Chuyên SPHN,30,Chuyên Trần Hưng Đạo,3,Collection,8,College Mathematic,1,Concours,1,Cono Sur,1,Contest,675,Correspondence,1,Cosmin Poahata,1,Crux,2,Czech-Polish-Slovak,28,Đà Nẵng,50,Đa Thức,2,Đại Số,20,Đắk Lắk,76,Đắk Nông,15,Danube,7,Đào Thái Hiệp,1,ĐBSCL,2,Đề Thi,1,Đề Thi HSG,2249,Đề Thi JMO,1,DHBB,30,Điện Biên,15,Định Lý,1,Định Lý Beaty,1,Đỗ Hữu Đức Thịnh,1,Do Thái,3,Doãn Quang Tiến,5,Đoàn Quỳnh,1,Đoàn Văn Trung,1,Đồng Nai,64,Đồng Tháp,63,Du Hiền Vinh,1,Đức,1,Dương Quỳnh Châu,1,Dương Tú,1,Duyên Hải Bắc Bộ,30,E-Book,31,EGMO,30,ELMO,19,EMC,11,Epsilon,1,Estonian,5,Euler,1,Evan Chen,1,Fermat,3,Finland,4,Forum Of Geometry,2,Furstenberg,1,G. Polya,3,Gặp Gỡ Toán Học,30,Gauss,1,GDTX,3,Geometry,14,GGTH,30,Gia Lai,40,Gia Viễn,2,Giải Tích Hàm,1,Giới hạn,2,Goldbach,1,Hà Giang,5,Hà Lan,1,Hà Nam,45,Hà Nội,255,Hà Tĩnh,91,Hà Trung Kiên,1,Hải Dương,70,Hải Phòng,57,Hậu Giang,14,Hélènne Esnault,1,Hilbert,2,Hình Học,33,HKUST,7,Hòa Bình,33,Hoài Nhơn,1,Hoàng Bá Minh,1,Hoàng Minh Quân,1,Hodge,1,Hojoo Lee,2,HOMC,5,HongKong,8,HSG 10,126,HSG 10 2010-2011,4,HSG 10 2011-2012,7,HSG 10 2012-2013,8,HSG 10 2013-2014,7,HSG 10 2014-2015,6,HSG 10 2015-2016,2,HSG 10 2016-2017,8,HSG 10 2017-2018,4,HSG 10 2018-2019,4,HSG 10 2019-2020,7,HSG 10 2020-2021,3,HSG 10 2021-2022,4,HSG 10 2022-2023,11,HSG 10 2023-2024,1,HSG 10 Bà Rịa Vũng Tàu,2,HSG 10 Bắc Giang,1,HSG 10 Bạc Liêu,2,HSG 10 Bình Định,1,HSG 10 Bình Dương,1,HSG 10 Bình Thuận,4,HSG 10 Chuyên SPHN,5,HSG 10 Đắk Lắk,2,HSG 10 Đồng Nai,4,HSG 10 Gia Lai,2,HSG 10 Hà Nam,4,HSG 10 Hà Tĩnh,15,HSG 10 Hải Dương,10,HSG 10 KHTN,9,HSG 10 Nghệ An,1,HSG 10 Ninh Thuận,1,HSG 10 Phú Yên,2,HSG 10 PTNK,10,HSG 10 Quảng Nam,1,HSG 10 Quảng Trị,2,HSG 10 Thái Nguyên,9,HSG 10 Vĩnh Phúc,14,HSG 1015-2016,3,HSG 11,135,HSG 11 2009-2010,1,HSG 11 2010-2011,6,HSG 11 2011-2012,10,HSG 11 2012-2013,9,HSG 11 2013-2014,7,HSG 11 2014-2015,10,HSG 11 2015-2016,6,HSG 11 2016-2017,8,HSG 11 2017-2018,7,HSG 11 2018-2019,8,HSG 11 2019-2020,5,HSG 11 2020-2021,8,HSG 11 2021-2022,4,HSG 11 2022-2023,7,HSG 11 2023-2024,1,HSG 11 An Giang,2,HSG 11 Bà Rịa Vũng Tàu,1,HSG 11 Bắc Giang,4,HSG 11 Bạc Liêu,3,HSG 11 Bắc Ninh,2,HSG 11 Bình Định,12,HSG 11 Bình Dương,3,HSG 11 Bình Thuận,1,HSG 11 Cà Mau,1,HSG 11 Đà Nẵng,9,HSG 11 Đồng Nai,1,HSG 11 Hà Nam,2,HSG 11 Hà Tĩnh,12,HSG 11 Hải Phòng,1,HSG 11 Kiên Giang,4,HSG 11 Lạng Sơn,11,HSG 11 Nghệ An,6,HSG 11 Ninh Bình,2,HSG 11 Quảng Bình,12,HSG 11 Quảng Nam,1,HSG 11 Quảng Ngãi,9,HSG 11 Quảng Trị,3,HSG 11 Sóc Trăng,1,HSG 11 Thái Nguyên,8,HSG 11 Thanh Hóa,3,HSG 11 Trà Vinh,1,HSG 11 Tuyên Quang,1,HSG 11 Vĩnh Long,3,HSG 11 Vĩnh Phúc,11,HSG 12,668,HSG 12 2009-2010,2,HSG 12 2010-2011,39,HSG 12 2011-2012,44,HSG 12 2012-2013,58,HSG 12 2013-2014,53,HSG 12 2014-2015,44,HSG 12 2015-2016,37,HSG 12 2016-2017,46,HSG 12 2017-2018,55,HSG 12 2018-2019,43,HSG 12 2019-2020,43,HSG 12 2020-2021,52,HSG 12 2021-2022,35,HSG 12 2022-2023,42,HSG 12 2023-2024,23,HSG 12 2023-2041,1,HSG 12 An Giang,8,HSG 12 Bà Rịa Vũng Tàu,13,HSG 12 Bắc Giang,18,HSG 12 Bạc Liêu,3,HSG 12 Bắc Ninh,13,HSG 12 Bến Tre,19,HSG 12 Bình Định,17,HSG 12 Bình Dương,8,HSG 12 Bình Phước,9,HSG 12 Bình Thuận,8,HSG 12 Cà Mau,7,HSG 12 Cần Thơ,7,HSG 12 Cao Bằng,5,HSG 12 Chuyên SPHN,11,HSG 12 Đà Nẵng,3,HSG 12 Đắk Lắk,21,HSG 12 Đắk Nông,1,HSG 12 Điện Biên,3,HSG 12 Đồng Nai,20,HSG 12 Đồng Tháp,18,HSG 12 Gia Lai,14,HSG 12 Hà Nam,5,HSG 12 Hà Nội,17,HSG 12 Hà Tĩnh,16,HSG 12 Hải Dương,16,HSG 12 Hải Phòng,20,HSG 12 Hậu Giang,4,HSG 12 Hòa Bình,10,HSG 12 Hưng Yên,10,HSG 12 Khánh Hòa,4,HSG 12 KHTN,26,HSG 12 Kiên Giang,12,HSG 12 Kon Tum,3,HSG 12 Lai Châu,4,HSG 12 Lâm Đồng,11,HSG 12 Lạng Sơn,8,HSG 12 Lào Cai,17,HSG 12 Long An,18,HSG 12 Nam Định,7,HSG 12 Nghệ An,13,HSG 12 Ninh Bình,12,HSG 12 Ninh Thuận,7,HSG 12 Phú Thọ,18,HSG 12 Phú Yên,13,HSG 12 Quảng Bình,14,HSG 12 Quảng Nam,11,HSG 12 Quảng Ngãi,6,HSG 12 Quảng Ninh,20,HSG 12 Quảng Trị,10,HSG 12 Sóc Trăng,4,HSG 12 Sơn La,5,HSG 12 Tây Ninh,6,HSG 12 Thái Bình,11,HSG 12 Thái Nguyên,13,HSG 12 Thanh Hóa,17,HSG 12 Thừa Thiên Huế,19,HSG 12 Tiền Giang,3,HSG 12 TPHCM,13,HSG 12 Tuyên Quang,3,HSG 12 Vĩnh Long,7,HSG 12 Vĩnh Phúc,20,HSG 12 Yên Bái,6,HSG 9,573,HSG 9 2009-2010,1,HSG 9 2010-2011,21,HSG 9 2011-2012,42,HSG 9 2012-2013,41,HSG 9 2013-2014,35,HSG 9 2014-2015,41,HSG 9 2015-2016,38,HSG 9 2016-2017,42,HSG 9 2017-2018,45,HSG 9 2018-2019,41,HSG 9 2019-2020,18,HSG 9 2020-2021,50,HSG 9 2021-2022,53,HSG 9 2022-2023,55,HSG 9 2023-2024,15,HSG 9 An Giang,9,HSG 9 Bà Rịa Vũng Tàu,8,HSG 9 Bắc Giang,14,HSG 9 Bắc Kạn,1,HSG 9 Bạc Liêu,1,HSG 9 Bắc Ninh,12,HSG 9 Bến Tre,9,HSG 9 Bình Định,11,HSG 9 Bình Dương,7,HSG 9 Bình Phước,13,HSG 9 Bình Thuận,5,HSG 9 Cà Mau,2,HSG 9 Cần Thơ,4,HSG 9 Cao Bằng,2,HSG 9 Đà Nẵng,11,HSG 9 Đắk Lắk,12,HSG 9 Đắk Nông,3,HSG 9 Điện Biên,5,HSG 9 Đồng Nai,8,HSG 9 Đồng Tháp,10,HSG 9 Gia Lai,9,HSG 9 Hà Giang,4,HSG 9 Hà Nam,10,HSG 9 Hà Nội,15,HSG 9 Hà Tĩnh,13,HSG 9 Hải Dương,16,HSG 9 Hải Phòng,8,HSG 9 Hậu Giang,6,HSG 9 Hòa Bình,4,HSG 9 Hưng Yên,11,HSG 9 Khánh Hòa,6,HSG 9 Kiên Giang,16,HSG 9 Kon Tum,9,HSG 9 Lai Châu,2,HSG 9 Lâm Đồng,14,HSG 9 Lạng Sơn,10,HSG 9 Lào Cai,4,HSG 9 Long An,10,HSG 9 Nam Định,9,HSG 9 Nghệ An,21,HSG 9 Ninh Bình,14,HSG 9 Ninh Thuận,4,HSG 9 Phú Thọ,13,HSG 9 Phú Yên,9,HSG 9 Quảng Bình,14,HSG 9 Quảng Nam,12,HSG 9 Quảng Ngãi,13,HSG 9 Quảng Ninh,17,HSG 9 Quảng Trị,10,HSG 9 Sóc Trăng,9,HSG 9 Sơn La,5,HSG 9 Tây Ninh,16,HSG 9 Thái Bình,11,HSG 9 Thái Nguyên,5,HSG 9 Thanh Hóa,12,HSG 9 Thừa Thiên Huế,9,HSG 9 Tiền Giang,7,HSG 9 TPHCM,11,HSG 9 Trà Vinh,2,HSG 9 Tuyên Quang,6,HSG 9 Vĩnh Long,12,HSG 9 Vĩnh Phúc,12,HSG 9 Yên Bái,5,HSG Cấp Trường,80,HSG Quốc Gia,113,HSG Quốc Tế,16,Hứa Lâm Phong,1,Hứa Thuần Phỏng,1,Hùng Vương,2,Hưng Yên,43,Huỳnh Kim Linh,1,Hy Lạp,1,IMC,26,IMO,58,IMT,2,IMU,2,India - Ấn Độ,47,Inequality,13,InMC,1,International,349,Iran,13,Jakob,1,JBMO,41,Jewish,1,Journal,30,Junior,38,K2pi,1,Kazakhstan,1,Khánh Hòa,30,KHTN,64,Kiên Giang,74,Kon Tum,24,Korea - Hàn Quốc,5,Kvant,2,Kỷ Yếu,46,Lai Châu,12,Lâm Đồng,47,Lăng Hồng Nguyệt Anh,1,Lạng Sơn,37,Langlands,1,Lào Cai,35,Lê Hải Châu,1,Lê Hải Khôi,1,Lê Hoành Phò,4,Lê Hồng Phong,5,Lê Khánh Sỹ,3,Lê Minh Cường,1,Lê Phúc Lữ,1,Lê Phương,1,Lê Viết Hải,1,Lê Việt Hưng,2,Leibniz,1,Long An,52,Lớp 10 Chuyên,709,Lớp 10 Không Chuyên,355,Lớp 11,1,Lục Ngạn,1,Lượng giác,1,Lưu Giang Nam,2,Lưu Lý Tưởng,1,Macedonian,1,Malaysia,1,Margulis,2,Mark Levi,1,Mathematical Excalibur,1,Mathematical Reflections,1,Mathematics Magazine,1,Mathematics Today,1,Mathley,1,MathLinks,1,MathProblems Journal,1,Mathscope,8,MathsVN,5,MathVN,1,MEMO,13,Menelaus,1,Metropolises,4,Mexico,1,MIC,1,Michael Atiyah,1,Michael Guillen,1,Mochizuki,1,Moldova,1,Moscow,1,MYM,25,MYTS,4,Nam Định,45,Nam Phi,1,National,276,Nesbitt,1,Newton,4,Nghệ An,73,Ngô Bảo Châu,2,Ngô Việt Hải,1,Ngọc Huyền,2,Nguyễn Anh Tuyến,1,Nguyễn Bá Đang,1,Nguyễn Đình Thi,1,Nguyễn Đức Tấn,1,Nguyễn Đức Thắng,1,Nguyễn Duy Khương,1,Nguyễn Duy Tùng,1,Nguyễn Hữu Điển,3,Nguyễn Minh Hà,1,Nguyễn Minh Tuấn,9,Nguyễn Nhất Huy,1,Nguyễn Phan Tài Vương,1,Nguyễn Phú Khánh,1,Nguyễn Phúc Tăng,2,Nguyễn Quản Bá Hồng,1,Nguyễn Quang Sơn,1,Nguyễn Song Thiên Long,1,Nguyễn Tài Chung,5,Nguyễn Tăng Vũ,1,Nguyễn Tất Thu,1,Nguyễn Thúc Vũ Hoàng,1,Nguyễn Trung Tuấn,8,Nguyễn Tuấn Anh,2,Nguyễn Văn Huyện,3,Nguyễn Văn Mậu,25,Nguyễn Văn Nho,1,Nguyễn Văn Quý,2,Nguyễn Văn Thông,1,Nguyễn Việt Anh,1,Nguyễn Vũ Lương,2,Nhật Bản,4,Nhóm $\LaTeX$,4,Nhóm Toán,1,Ninh Bình,61,Ninh Thuận,26,Nội Suy Lagrange,2,Nội Suy Newton,1,Nordic,21,Olympiad Corner,1,Olympiad Preliminary,2,Olympic 10,134,Olympic 10/3,6,Olympic 10/3 Đắk Lắk,6,Olympic 11,122,Olympic 12,52,Olympic 23/3,2,Olympic 24/3,10,Olympic 24/3 Quảng Nam,10,Olympic 27/4,24,Olympic 30/4,61,Olympic KHTN,8,Olympic Sinh Viên,78,Olympic Tháng 4,12,Olympic Toán,344,Olympic Toán Sơ Cấp,3,Ôn Thi 10,2,PAMO,1,Phạm Đình Đồng,1,Phạm Đức Tài,1,Phạm Huy Hoàng,1,Pham Kim Hung,3,Phạm Quốc Sang,2,Phan Huy Khải,1,Phan Quang Đạt,1,Phan Thành Nam,1,Pháp,2,Philippines,8,Phú Thọ,32,Phú Yên,42,Phùng Hồ Hải,1,Phương Trình Hàm,11,Phương Trình Pythagoras,1,Pi,1,Polish,32,Problems,1,PT-HPT,14,PTNK,64,Putnam,27,Quảng Bình,64,Quảng Nam,57,Quảng Ngãi,49,Quảng Ninh,60,Quảng Trị,42,Quỹ Tích,1,Riemann,1,RMM,14,RMO,24,Romania,38,Romanian Mathematical,1,Russia,1,Sách Thường Thức Toán,7,Sách Toán,70,Sách Toán Cao Học,1,Sách Toán THCS,7,Saudi Arabia - Ả Rập Xê Út,9,Scholze,1,Serbia,17,Sharygin,28,Shortlists,56,Simon Singh,1,Singapore,1,Số Học - Tổ Hợp,28,Sóc Trăng,36,Sơn La,22,Spain,8,Star Education,1,Stars of Mathematics,11,Swinnerton-Dyer,1,Talent Search,1,Tăng Hải Tuân,2,Tạp Chí,17,Tập San,3,Tây Ban Nha,1,Tây Ninh,37,Thái Bình,45,Thái Nguyên,61,Thái Vân,2,Thanh Hóa,69,THCS,2,Thổ Nhĩ Kỳ,5,Thomas J. Mildorf,1,Thông Tin Toán Học,43,THPT Chuyên Lê Quý Đôn,1,THPT Chuyên Nguyễn Du,9,THPTQG,16,THTT,31,Thừa Thiên Huế,56,Tiền Giang,30,Tin Tức Toán Học,1,Titu Andreescu,2,Toán 12,7,Toán Cao Cấp,3,Toán Rời Rạc,5,Toán Tuổi Thơ,3,Tôn Ngọc Minh Quân,2,TOT,1,TPHCM,158,Trà Vinh,10,Trắc Nghiệm,1,Trắc Nghiệm Toán,2,Trại Hè,39,Trại Hè Hùng Vương,30,Trại Hè Phương Nam,7,Trần Đăng Phúc,1,Trần Minh Hiền,2,Trần Nam Dũng,12,Trần Phương,1,Trần Quang Hùng,1,Trần Quốc Anh,2,Trần Quốc Luật,1,Trần Quốc Nghĩa,1,Trần Tiến Tự,1,Trịnh Đào Chiến,2,Trường Đông,23,Trường Hè,10,Trường Thu,1,Trường Xuân,3,TST,544,TST 2008-2009,1,TST 2010-2011,22,TST 2011-2012,23,TST 2012-2013,32,TST 2013-2014,29,TST 2014-2015,27,TST 2015-2016,26,TST 2016-2017,41,TST 2017-2018,42,TST 2018-2019,30,TST 2019-2020,34,TST 2020-2021,30,TST 2021-2022,38,TST 2022-2023,42,TST 2023-2024,23,TST An Giang,8,TST Bà Rịa Vũng Tàu,11,TST Bắc Giang,5,TST Bắc Ninh,11,TST Bến Tre,10,TST Bình Định,5,TST Bình Dương,7,TST Bình Phước,9,TST Bình Thuận,9,TST Cà Mau,7,TST Cần Thơ,6,TST Cao Bằng,2,TST Đà Nẵng,8,TST Đắk Lắk,12,TST Đắk Nông,2,TST Điện Biên,2,TST Đồng Nai,13,TST Đồng Tháp,12,TST Gia Lai,4,TST Hà Nam,8,TST Hà Nội,12,TST Hà Tĩnh,15,TST Hải Dương,11,TST Hải Phòng,13,TST Hậu Giang,1,TST Hòa Bình,4,TST Hưng Yên,10,TST Khánh Hòa,8,TST Kiên Giang,11,TST Kon Tum,6,TST Lâm Đồng,12,TST Lạng Sơn,3,TST Lào Cai,4,TST Long An,6,TST Nam Định,8,TST Nghệ An,7,TST Ninh Bình,11,TST Ninh Thuận,4,TST Phú Thọ,13,TST Phú Yên,5,TST PTNK,15,TST Quảng Bình,12,TST Quảng Nam,7,TST Quảng Ngãi,8,TST Quảng Ninh,9,TST Quảng Trị,10,TST Sóc Trăng,5,TST Sơn La,7,TST Thái Bình,6,TST Thái Nguyên,8,TST Thanh Hóa,9,TST Thừa Thiên Huế,4,TST Tiền Giang,6,TST TPHCM,14,TST Trà Vinh,1,TST Tuyên Quang,1,TST Vĩnh Long,7,TST Vĩnh Phúc,7,TST Yên Bái,8,Tuyên Quang,14,Tuyển Sinh,4,Tuyển Sinh 10,1064,Tuyển Sinh 10 An Giang,18,Tuyển Sinh 10 Bà Rịa Vũng Tàu,22,Tuyển Sinh 10 Bắc Giang,19,Tuyển Sinh 10 Bắc Kạn,3,Tuyển Sinh 10 Bạc Liêu,9,Tuyển Sinh 10 Bắc Ninh,15,Tuyển Sinh 10 Bến Tre,34,Tuyển Sinh 10 Bình Định,19,Tuyển Sinh 10 Bình Dương,12,Tuyển Sinh 10 Bình Phước,21,Tuyển Sinh 10 Bình Thuận,15,Tuyển Sinh 10 Cà Mau,5,Tuyển Sinh 10 Cần Thơ,10,Tuyển Sinh 10 Cao Bằng,2,Tuyển Sinh 10 Chuyên SPHN,19,Tuyển Sinh 10 Đà Nẵng,18,Tuyển Sinh 10 Đại Học Vinh,13,Tuyển Sinh 10 Đắk Lắk,21,Tuyển Sinh 10 Đắk Nông,7,Tuyển Sinh 10 Điện Biên,5,Tuyển Sinh 10 Đồng Nai,18,Tuyển Sinh 10 Đồng Tháp,23,Tuyển Sinh 10 Gia Lai,10,Tuyển Sinh 10 Hà Giang,1,Tuyển Sinh 10 Hà Nam,16,Tuyển Sinh 10 Hà Nội,80,Tuyển Sinh 10 Hà Tĩnh,19,Tuyển Sinh 10 Hải Dương,17,Tuyển Sinh 10 Hải Phòng,15,Tuyển Sinh 10 Hậu Giang,3,Tuyển Sinh 10 Hòa Bình,15,Tuyển Sinh 10 Hưng Yên,12,Tuyển Sinh 10 Khánh Hòa,12,Tuyển Sinh 10 KHTN,21,Tuyển Sinh 10 Kiên Giang,31,Tuyển Sinh 10 Kon Tum,6,Tuyển Sinh 10 Lai Châu,6,Tuyển Sinh 10 Lâm Đồng,10,Tuyển Sinh 10 Lạng Sơn,6,Tuyển Sinh 10 Lào Cai,10,Tuyển Sinh 10 Long An,18,Tuyển Sinh 10 Nam Định,21,Tuyển Sinh 10 Nghệ An,23,Tuyển Sinh 10 Ninh Bình,20,Tuyển Sinh 10 Ninh Thuận,10,Tuyển Sinh 10 Phú Thọ,18,Tuyển Sinh 10 Phú Yên,12,Tuyển Sinh 10 PTNK,37,Tuyển Sinh 10 Quảng Bình,12,Tuyển Sinh 10 Quảng Nam,15,Tuyển Sinh 10 Quảng Ngãi,13,Tuyển Sinh 10 Quảng Ninh,12,Tuyển Sinh 10 Quảng Trị,7,Tuyển Sinh 10 Sóc Trăng,17,Tuyển Sinh 10 Sơn La,5,Tuyển Sinh 10 Tây Ninh,15,Tuyển Sinh 10 Thái Bình,17,Tuyển Sinh 10 Thái Nguyên,18,Tuyển Sinh 10 Thanh Hóa,27,Tuyển Sinh 10 Thừa Thiên Huế,24,Tuyển Sinh 10 Tiền Giang,14,Tuyển Sinh 10 TPHCM,23,Tuyển Sinh 10 Trà Vinh,6,Tuyển Sinh 10 Tuyên Quang,3,Tuyển Sinh 10 Vĩnh Long,12,Tuyển Sinh 10 Vĩnh Phúc,22,Tuyển Sinh 2008-2009,1,Tuyển Sinh 2009-2010,1,Tuyển Sinh 2010-2011,6,Tuyển Sinh 2011-2012,20,Tuyển Sinh 2012-2013,65,Tuyển Sinh 2013-2014,77,Tuyển Sinh 2013-2044,1,Tuyển Sinh 2014-2015,81,Tuyển Sinh 2015-2016,64,Tuyển Sinh 2016-2017,72,Tuyển Sinh 2017-2018,126,Tuyển Sinh 2018-2019,61,Tuyển Sinh 2019-2020,90,Tuyển Sinh 2020-2021,59,Tuyển Sinh 2021-202,1,Tuyển Sinh 2021-2022,69,Tuyển Sinh 2022-2023,113,Tuyển Sinh 2023-2024,49,Tuyển Sinh Chuyên SPHCM,7,Tuyển Sinh Yên Bái,6,Tuyển Tập,45,Tuymaada,6,UK - Anh,16,Undergraduate,69,USA - Mỹ,62,USA TSTST,6,USAJMO,12,USATST,8,USEMO,4,Uzbekistan,1,Vasile Cîrtoaje,4,Vật Lý,1,Viện Toán Học,6,Vietnam,4,Viktor Prasolov,1,VIMF,1,Vinh,32,Vĩnh Long,41,Vĩnh Phúc,86,Virginia Tech,1,VLTT,1,VMEO,4,VMF,12,VMO,58,VNTST,25,Võ Anh Khoa,1,Võ Quốc Bá Cẩn,26,Võ Thành Văn,1,Vojtěch Jarník,6,Vũ Hữu Bình,7,Vương Trung Dũng,1,WFNMC Journal,1,Wiles,1,Xác Suất,1,Yên Bái,25,Yên Thành,1,Zhautykov,14,Zhou Yuan Zhe,1,
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MOlympiad.NET: Mathematics and Youth Magazine Problems 2005
Mathematics and Youth Magazine Problems 2005