## $hide=mobile$type=ticker$c=12$cols=3$l=0$sr=random$b=0 # ĐẶT MUA TẠP CHÍ / PURCHASE JOURNALS ### Issue 403 1. Given the sum $$S=\frac{5}{1.2 .3}+\frac{8}{2.3 .4}+\frac{11}{3.4 .5}+\ldots+\frac{6026}{2008.2009 .2010}.$$ Compare$S$with$2$. 2. Let$A B C$be a triangle with$\widehat{B A C}=50^{\circ}$,$\widehat{A B C}=72^{\circ}$. Outside of the triangle$A B C,$draw a triangle$B D C$such that$\widehat{C B D}=28^{\circ}$;$\widehat{B C D}=22^{\circ}$. Find the measure of the angle$A D B$. 3. Find all possible pair of integers$x$,$y$satisfying the following condition $$x^{2}+y^{2}=(x-y)(x y+2)+9$$ 4. Given$a, b, c, d \in[0 ; 1]$satisfies the following condition $$a+b+c+d=x+y+z+t=1.$$ Prove the inequality $$a x+b y+c z+d t \geq 54 a b c d.$$ 5. Let$A B C$be a triangle with$\widehat{B A C}=45^{\circ}$. The attitudes$B D$and$C E$intersect at$H .$Let$I$be a midpoint of$D E .$Prove that the line$H I$goes through the centroid of the triangle$A B C$. 6. Solve the equation $$\sqrt{x}+\sqrt[4]{x}+4 \sqrt{17-x}+8 \sqrt[4]{17-x}=34.$$ 7. A number is said to be an interesting number if it has$10$digits, all are distinct, and is a multiple of$11111$. How many interesting numbers are there? 8. Let$A_{1} A_{2} A_{3} \ldots A_{n}$be a convex polygon$(n \geq 3)$on the plane$(P)$and let$S$be a point outside$(P)$. Another plane$(\alpha)$intersects the sides$S A_{1}, S A_{2}, \ldots, S A_{n}$at$B_{1}, B_{2},\ldots, B_{n}$respectively such that $$\frac{S A_{1}}{S B_{1}}+\frac{S A_{2}}{S B_{2}}+\ldots+\frac{S A_{n}}{S B_{n}}=a$$ where$a$is a given positive number. Prove that such a plane$(\alpha)$always contains a fixed point. 9. Two circles$\omega_{1}$,$\omega_{2}$intersect at points$A$,$B$.$C D$is a common tangent line of$\omega_{1}$,$\omega_{2}(C \in \omega_{1}, D \in \omega_{2}$) where point$B$is closer to$C D$than point$A$.$C B$cuts$A D$at$E$,$D B$cuts$CA$at$F$and$E F$cuts$A B$at$N$.$K$is the orthogonal projection of$N$onto$C D$. a) Prove that$\widehat{C A B}=\widehat{D A K}$. b) Let$O$be the circumcenter of the triangle$A C D$and$H$is the orthocenter of the triangle$K E F .$Prove that$O$,$B$,$H$are collinear 10. Let$\left(x_{n}\right)$be the sequence where $$x_{1}=5,\quad x_{n+1}=\frac{x_{n}^{2010}+3 x_{n}+16}{x_{n}^{2009}-x_{n}+11},\,n=1,2, \ldots$$ For each positive number$n,$put$\displaystyle y_{n}=\sum_{i=1}^{n} \frac{1}{x_{i}^{2009}+7}$. Determine$\displaystyle \lim_{n\to\infty}y_{n}$. 11. Find all functions$f: \mathbb{R} \rightarrow \mathbb{R}$satisfy the following equation $$f(f(x-y))=f(x) \cdot f(y)+f(x)-f(y)-x y,\,\forall x, y \in \mathbb{R}.$$ 12. For each$n \in \mathbb{N}$, let$a_{n}$be a number of bijections$f:\{1,2, \ldots, n\} \rightarrow\{1,2, \ldots, n\}$such that$f(f(k))=k$for all$k \in\{1,2, \ldots, n\}$. a) Prove that$a_{n}$is an even number for every$n \geq 2$. b) Prove that for every$n \geq 10$and$n$is divisible by$3$then$a_{n}-a_{n-9}$is divisible by$3$. ### Issue 404 1. Given seven distinct prime numbers$a$,$b$,$c$,$a+b+c$,$a+b-c$,$a-b+c$,$-a+b+c$in which the sum of two of three numbers$a$,$b$,$c$equals$800$. Let$d$be the difference between the largest and the smallest number among these seven integers. What is the maximum value of$d ?$2. A triangle$A B C$has sides$A B=2cm$,$A C=4cm$and median$A M=\sqrt{3}cm$. Find the measure of the angle$B A C$, the length of side$B C$and the area of triangle$A B C$. 3. Find the largest natural number$k$so that$n^{5}-2011 n$is divisible by$k$for all natural number$n$. 4. Solve the equation $$\left(x^{4}-625\right)^{2}-100 x^{2}-1=0$$ 5. Let$A B C$be an acute triangle$(A B \neq A C)$inscribed in the circle$(O),$and$H$is its orthocenter. Let$d$be an arbitrary line passing through$H$. Draw the line$d^{\prime}$symmetric to$d$through$B C$. Find the position of the line$d$so that$d^{\prime}$touches the circumcircle$(O)$. 6. Find the largest constant$k$such that $$\frac{a^{2}}{b}+\frac{b^{2}}{c}+\frac{c^{2}}{a} \geq k\left(a^{2}+b^{2}+c^{2}\right)$$ for all positive real numbers$a$,$b$, and$c$whose sum equals$1 .$7. Let$A B C$be a triangle inscribed in the circle$(O)$, and let$G$be its centroid;$D$,$E$, and$F$are the circumcenters of triangles$G B C$,$G C A$,$G A B$respectively. Prove that$O$is the centroid of$D E F$. 8. Solve the equation $$\sqrt[3]{\cos 5 x+2 \cos x}-\sqrt[3]{2 \cos 5 x+\cos x}=2 \sqrt[3]{\cos x}(\cos 4 x-\cos 2 x).$$ 9. Let$x$,$y$,$z$be real numbers such that$x \geq 1$,$y \geq 2$,$z \geq 3$and $$\frac{1}{x+\sqrt{x-1}}+\frac{2}{y+\sqrt{y-2}}+\frac{3}{z+\sqrt{z-3}}=12$$ Find the maximum and minimum value of the function$f(x, y, z)=x+y+z$. 10. Let$\left(x_{n}\right)$be a sequence given by $$x_{1}=\frac{5}{2},\quad x_{n+1}=\sqrt{x_{n}^{3}-12 x_{n}+\frac{20 n+21}{n+1}},\,\forall n \in \mathbb{N}^{*}.$$ Prove that the sequence$\left(x_{n}\right)$converges and find its limit. 11. Find all functions$f: \mathbb{R} \rightarrow(0 ; 2011]$such that $$f(x) \leq 2011\left(2-\frac{2011}{f(y)}\right),\,\forall x>y.$$ 12. Given four points$A_{I}(i=1,2,3,4),$no three of them are colinear and a point$M$so that$A_{i}(i=1,2,3,4)$and$M$do not lie on the same circle. Let$T_{i}$be a triangle having$A_{j}(j=1,2,3,4 ; j \neq i)$as its vertices,$C_{i}$is the circle (or the line) passing through the feet of the projections through$M$onto three sides (or extended sides) of triangle$T_{i}$. Prove that$C_{I}(i=1,2,3,4)$have a common point. ### Issue 405 1. Which of the following two numbers is greater? $$A=\frac{326}{1955}+\frac{988}{1975}+\frac{662}{1985},\quad B=\dfrac{3951}{3950}+\dfrac{1}{5955}+\dfrac{1}{11730}.$$ 2. Let$A B C$be an isosceles triangle with$\widehat{B A C}=96^{\circ} .$A point$M$is inside the triangle such that$\widehat{M B C}=12^{\circ}$,$\widehat{M C B}=24^{\circ} .$Prove that$M A=M C$. 3. Find the maximum value of the expression$P=\max \{a, b, c\}-\min \{a, b, c\}$where$a, b, c$are real numbers satisfying the condition $$a+b+c=a^{3}+b^{3}+c^{3}-3 a b c=2.$$ 4. Solve the equation $$21 x-25+2 \sqrt{x-2}=19 \sqrt{x^{2}-x+2}+\sqrt{x+1}$$ 5.$A B C$is a triangle inscribed the circle$(O)$with$\widehat{B A C}=60^{\circ}$,$A K$is the angle-bisector of$\widehat{B A C}$($K$is on the circle$(O)$). Let$F$be the midpoint of$A K,$the ray$O F$meets the altitude$C E$of triangle$A B C$at$H .$Prove that$B H$is perpendicular to$A C$. 6. Find the minimum value of the expression $$P=\left(5 a+\frac{2}{b+c}\right)^{3}+\left(5 b+\frac{2}{c+a}\right)^{3}+\left(5 c+\frac{2}{a+b}\right)^{3}$$ where$a, b, c$are positive real numbers satisfying$a^{2}+b^{2}+c^{2}=3$7.$OABC$is a trirectangular tetrahedron at vertex$O$.$O H$is the altitude from$O$of tetrahedron. Let$R$be the circumradius of triangle$A B C .$Prove that$O H \leq \dfrac{R \sqrt{2}}{2} .$When does equality occur? 8. a) Find all distinct permutations of the word$TOANHOCTUOITRE$. b) How many permutations are there that has three consecutive$T - TTT$? c) How many permutations are there without adjacent$T$s? 9. Let$k$be a positive integer,$\alpha$is an arbitrary real number. Find the limit of sequence$\left(a_{n}\right)$where $$a_{n}=\frac{\left[1^{k} \cdot \alpha\right]+\left[2^{k} \cdot \alpha\right]+\ldots+\left[n^{k} \cdot \alpha\right]}{n^{k+1}},\, n=1,2, \ldots$$ here the notation$[x]$is the largest integer that does not exceed$x .$10. Find all functions$f: \mathbb{N} \rightarrow \mathbb{N}$which satisfy the following conditions •$f$is strictly increasing; •$f(f(n))=4 n+9$for all$n \in \mathbb{N}^{*}$•$f(f(n)-n)=2 n+9$for all$n \in \mathbb{N}^{*}$11. Does there exist a positive integer$n \geq 2$so that $$f(x)=1+4 x+4 x^{2}+\ldots+4 x^{2 n}$$ is a perfect square polynomial? 12. Let$A B C$be a triangle inscribed the circle$(O)$and$A^{\prime}$is a fixed point on$(O)$.$P$moves on$B C$,$K$belongs to$A C$so that$P K$is always parallel to a fixed line$d$. The circumcircle of triangle$A P K$cuts the circle$(O)$at a second point$E$.$A E$cuts$B C$at$M$.$A^{\prime} P$cuts the circle$(O)$at a second point$N .$Prove that the line$M N$passes through a fixed point. ### Issue 406 ### Issue 407 1. Denote$T(a)$the number of digits of the natural number$a$. If$T\left(5^{n}\right)-T\left(2^{n}\right)$is even, is$n$necessarily odd or even? 2. A triangle$A B C$has$\widehat{B A C}<90^{\circ}$and$H A=B C$where$H$is its orthocenter. Find the measure of angle$B A C$. 3. Find all pair of integers$x, y$such that $$7^{x}+24^{x}=y^{2}$$ 4. Let$x, y, z$be arbitrary positive real numbers, prove the inequality $$\frac{x^{2}-z^{2}}{y+z}+\frac{z^{2}-y^{2}}{x+y}+\frac{y^{2}-x^{2}}{z+x} \geq 0.$$ When does equality occur? 5. From point$M$outside circle$(O),$draw tangent$M A$and secant$M B C$($B$is between$M$and$C$). Let$H$be the projection of$A$onto$M O, K$is the intersection of segment$M O$with$(O)$. Prove that a) The quadrilateral$O H B C$is cyclic. b)$B K$is the internal angle-bisector of angle$H B M$6. Solve the system of equations $$\begin{cases}\sqrt{\dfrac{x^{2}+y^{2}}{2}}+\sqrt{\dfrac{x^{2}+x y+y^{2}}{3}} &=x+y \\ x \sqrt{2 x y+5 x+3} &=4 x y-5 x-3 \end{cases}$$ 7. Let$\left(x_{n}\right)$be a sequence defined by $$3 x_{n+1}=x_{n}^{3}-2,\,n=1,2, \ldots.$$ For what values of$x_{1}$does the sequence$\left(x_{n}\right)$converge? Determine this limit when it converges. 8.$S . A B C$is a tetrahedron with isosceles perpendicular to plane$(A B C)$.$D$is the midpoint of$B C .$Let$\alpha$is the angle between edge$S B$and plane$(A B C)$;$\beta$is the angle between edge$S B$and plane$(S A D)$. Prove that $$\cos ^{2} \alpha+\cos ^{2} \beta>1$$ 9. Let$x, y, z$be positive real numbers such that$x^{2}+y^{2}+z^{2}+2 x y z=1 .$Prove the inequality $$8(x+y+z)^{3} \leq 10\left(x^{3}+y^{3}+z^{3}\right) + 11(x+y+z)(1+4 x y z)-12 x y z.$$ 10. Let$p$be an odd prime,$n$is a positive integer so that$(n, p)=1$. Find the number of tuples$\left(a_{1}, a_{2}, \ldots, a_{p-1}\right)$such that the sum$\displaystyle\sum_{k=1}^{p-1} k a_{k}$is divisible by$p,$and$a_{1}, a_{2}, \ldots, a_{p-1}$are natural numbers which do not exceed$n-1$. 11. Find all the functions$f$which is defined on$\mathbb{R},$take value on$\mathbb{R}$and satisfying the equation$f(x+y+f(y))=f(f(x))+2 y,$for all real numbers$x$,$y$. 12. Let$p, r, r_{a}, r_{b}, r_{c}$be semiperimeter, inradius, and exradius opposite angles$A$,$B$,$C$of triangle$A B C$having side lengths$B C=a$,$C A=b$,$A B=c$. Prove the inequality $$\sqrt{a b}+\sqrt{b c}+\sqrt{c a} \geq p+\sqrt{r r_{a}}+\sqrt{r r_{b}}+\sqrt{r r_{c}}$$ When does equality hold? ### Issue 408 1. How many integers$n$are there such that$-1964 \leq n \leq 2011$and the fraction$\dfrac{n^{2}+2}{n+9}$is reducible? 2. Let$a_{1}, a_{2}, a_{3}, \ldots$be a sequence satisfying the conditions $$a_{2}=3,\, a_{50}=300,\quad a_{n}+a_{n+1}=a_{n+2},\quad \forall n \geq 1 .$$ Find the sum of the first$48$terms$S=a_{1}+a_{2}+\ldots+a_{48}$. 3. Let$p$be a prime number. Let$x, y$be nonzero natural numbers such that$\dfrac{x^{2}+p y^{2}}{x y}$is also a natural number. Prove that $$\frac{x^{2}+p y^{2}}{x y}=p+1$$ 4. Solve the system of equations $$\begin{cases}x-2 \sqrt{y+1} &=3 \\ x^{3}-4 x^{2} \sqrt{y+1}-9 x-8 y &=-52-4 x y \end{cases}$$ 5. Let$A B$be a fixed line segment. Point$M$is such that$M A B$is an acute triangle. Let$H$be the orthocenter of$M A B$,$I$is the midpoint of$A B$,$D$is the projection of$H$onto$MI$. Prove that the product$M I$.$D I$does not depend on the position of$M$. 6. Let$a, b, c$be real numbers such that$\sin a+\sin b+\sin c \geq \dfrac{3}{2}$. Prove the inequality $$\sin \left(a-\frac{\pi}{6}\right)+\sin \left(b-\frac{\pi}{6}\right)+\sin \left(c-\frac{\pi}{6}\right) \geq 0$$ 7. Denote by$[x]$the largest integer not exceeding$x$. Solve the equation $$x^{2}-(1+[x]) x+2011=0.$$ 8. Let$E$be the center of the nine-point circle (the Euler's circle) of triangle$A B C$with edge-lengths$B C=a$,$A C=b$,$A B=c$;$E_{1}$,$E_{2}$,$E_{3}$are respectively the projections of$E$onto$B C$,$C A$,$A B$and let$R$be the circumradius of triangle$A B C$. Prove that $$\frac{S_{E_{1} E_{2} E_{3}}}{S_{A B C}}=\frac{a^{2}+b^{2}+c^{2}}{16 R^{2}}-\frac{5}{16}$$ 9. Find the minimum value of the expression $$\tan B+\tan C-\tan A \tan A+\tan C-\tan B \quad \tan A+\tan B-\tan C$$ where$A$,$B$,$C$are three angles of an acute triangle$A B C$and$C \geq A$10. a) Prove that for each positive integer$n,$the equation $$x+x^{2}+x^{3}+\ldots+2011 x^{2 n+1}=2009$$ has a unique real root. b) Let$x_{n}$be denote the real solution in part a). Prove that$0<x_{n}<\dfrac{2010}{2011}$. 11. Let$\left(u_{n}\right)$be a sequence given by $$u_{1}=2011,\quad u_{n+1}=\frac{\pi}{8}\left(\cos u_{n}+\frac{\cos 2 u_{n}}{2}+\frac{\cos 3 u_{n}}{3}\right),\, \forall n \geq 1.$$ Prove that the sequence$\left(u_{n}\right)$has a finite limit. 12. Let$A_{1} B_{1} C_{1} D_{1}$and$A_{2} B_{2} C_{2} D_{2}$be two squares in opposite direction (that is, if the vertices$A_{1}$,$B_{1}$,$C_{1}$,$D_{1}$are in clockwise order, then$A_{2}$,$B_{2}$,$C_{2}$,$D_{2}$are ordered counterclockwise) with centers$O_{1}$,$O_{2}$suppose that$D_{2}$,$D_{1}$are respectively in$A_{1} B_{1}$,$A_{2} B_{2}$. Prove that the lines$B_{1} B_{2}$,$C_{1} C_{2}$and$O_{1} O_{2}$are concurrent. ### Issue 409 1. Without taking common denominator, find the integer$x$given that $$\left(\frac{2009}{2010}+\frac{2010}{2011}+\frac{2011}{2009}\right) \cdot(x-2011)>3 x-6033$$ 2. In a triangle$A B C$,$M$,$N$,$P$are midpoints of sides$B C$,$C A$and$A B$respectively. Choose the points$A_{1}$,$A_{2}$,$B_{1}$,$B_{2}$,$C_{1}$,$C_{2}$on the opposite rays of rays$B A$,$C A$,$C B$,$A B$,$A C$and$B C$respectively so that$B A_{1}=C A_{2}$,$C B_{1}=A B_{2}$,$A C_{1}=B C_{2} . A_{0}, B_{0}, C_{0}$are midpoints of$A_{1} A_{2}, B_{1} B_{2}, C_{1} C_{2}$respectively. Prove that the lines$A_{0} M, B_{0} N, C_{0} P$meet at a common point. 3. Let$b$be a positive integer with the following properties •$b$equals a sum of three squares. •$b$possess a divisor of the form$a=3 k^{2}+3 k+1(k \in \mathbb{N})$. Prove that$b^{n}$is also a sum of three squares for any positive integer$n$. 4. Given three non-negative numbers$a, b, c$, prove the inequality $$a+b+c \geq \frac{a-b}{b+2}+\frac{b-c}{c+2}+\frac{c-a}{a+2}.$$ When does equality hold? 5. Let$A B C$be a triangle with circumcircle$(O)$,$I$is the midpoint of side$B C$.$M$is chosen on$I C$(differ from both$C$and$I$).$A M$meets$(O)$at$D$. Point$E$is on$B D$such that$\widehat{B M E}=\widehat{M A I}$.$E M$and$D C$intersect at$F$. Prove that $$\frac{C F}{C D}=\frac{B E}{B D}$$ 6. Solve for$x$$$\sqrt{\frac{x+2}{2}}-1=\sqrt[3]{3(x-3)^{2}}+\sqrt[3]{9(x-3)}$$ 7. Triangle$A B C$is inscribed in a fixed circle$(O)$. The medians from$A$,$B$,$C$meets$(O)$at$A_{1}$,$B_{1}$,$C_{1}$respectively. Which triangle makes the value of the expression $$p=\frac{A A_{1}^{2}+B B_{1}^{2}+C C_{1}^{2}}{A B^{2}+B C^{2}+C A^{2}}$$ minimum possible? 8. In a triangle$A B C$, prove that $$\frac{\sin A \cdot \sin B}{\sin ^{2} \frac{C}{2}}+\frac{\sin B \cdot \sin C}{\sin ^{2} \frac{A}{2}}+\frac{\sin C \cdot \sin A}{\sin ^{2} \frac{B}{2}} \geq 9$$ 9. Let$A B C$be a triangle with points$A'$,$B'$,$C'$on sides$B C$,$C A$and$A B$respectively such that $$\frac{A^{\prime} B}{A^{\prime} C}=\frac{B^{\prime} C}{B^{\prime} A}=\frac{C^{\prime} A}{C^{\prime} B}.$$$A A^{\prime}$and$B B^{\prime}$meet at$D$,$B B$' meets$C C$' at$E$and$F$is the intersection of$C C'$and$A A '$. Parallel lines to$A A'$,$B B'$,$C C'$through point$O$in the interior of$A B C$meet$B C$,$C A$,$A B$at$A_{1}$,$B_{1}$,$C_{1}$respectively. Prove that for any point$M$$$A D\left(M A_{1}-O A_{1}\right)+B E\left(M B_{1}-O B_{1}\right)+C F\left(M C_{1}-O C_{1}\right) \geq 0$$ 10. Given$P=(n+1)^{7}-n^{7}-1(n \in \mathbb{N})$. Prove that there are infinitely many natural numbers$n$so that$P$is a perfect square. 11. Determine all functions$f: \mathbb{R} \rightarrow \mathbb{R}$, continuous on$\mathbb{R}$such that $$f(x y)+f(x+y)=f(x y+x)+f(y),\, \forall x, y \in \mathbb{R}.$$ 12.$f: \mathbb{R} \rightarrow \mathbb{R}$is a function with the following properties •$f(1)=2011$, •$f(x+1) f(x)=(f(x))^{2}+f(x)-1, \forall x \in \mathbb{R}$. Let$\displaystyle S_{1}=\sum_{i=1}^{n} \frac{1}{f(i)-1}$,$\displaystyle S_{2}=\sum_{i=1}^{n} \frac{1}{f(i)+1}$. Find$\displaystyle\lim_{n \rightarrow+\infty}\left(S_{1}+S_{2}\right)$. ### Issue 410 1. Find all pairs of coprime positive integers$x, y$so that $$\frac{x+y}{x^{2}+y^{2}}=\frac{7}{25}$$ 2. Let$A B C$be an equilateral triangle whose altitudes$A H$,$B K$intersect at$G .$The angle-bisector of angle$B K H$meets$C G$,$A H$,$B C$at$M$,$N$,$P$respectively. Prove that$K M=N P$. 3. Find the minimum value of the expression$S=2011 c a-a b-b c$where$a, b, c$satisfy$a^{2}+b^{2}+c^{2} \leq 2$. 4. Let$A B C$be an isosceles right triangle with right angle at$A .$Let$M$,$N$,$O$be respectively the midpoints of$A B$,$A C$,$B C$. The line perpendicular to$C M$from$O$cuts$M N$at$G .$Compare the lengths of the two segments$G M$and$G N$. 5. Solve the equation $$\sqrt{7 x^{2}+25 x+19}-\sqrt{x^{2}-2 x-35}=7 \sqrt{x+2}$$ 6. Let$A B C$be a triangle. Let$A M$,$B N$,$C P$be its internal angle-bisectors ($M \in B C$,$N \in C A$,$P \in A B$). Find the measure of angle$B A C$so that$P M$is perpendicular to$N M$7. Solve the equation $$(\sin x-2)\left(\sin ^{2} x-\sin x+1\right)=3 \sqrt[3]{3 \sin x-1}+1.$$ 8. Find all values of$a, b$so that the equation $$x^{4}+a x^{3}+b x^{2}+a x+1=0$$ has at least one solution and the sum$a^{2}+b^{2}$is smallest possible. 9. Let$a, b, c$be positive numbers. Prove that $$\left(a^{2012}-a^{2010}+3\right)\left(b^{2012}-b^{2010}+3\right)\left(c^{2012}-c^{2010}+3\right) \geq 9(a b+b c+c a).$$ When does equality occur? 10. Let$A B C$be a triangle. An arbitrary line cuts the lines$B C$,$C A$,$A B$at$M$,$N$,$P$respectively. Let$X$,$Y$,$Z$be respectively the centroids of triangles$A N P$,$B P M$,$C M N$. Prove that $$S_{X Y Z}=\frac{2}{9} S_{A B C}$$ 11. Let$\left(a_{n}\right)\left(n \in \mathbb{N}^{*}\right)$be a sequence given by $$a_{1}=0 ,\, a_{2}=38,\, a_{3}=-90,\quad a_{n+1}=19 a_{n-1}-30 a_{n-2},\, \forall n \geq 3.$$ Prove that$a_{2011}$is divisible by 2011 . 12. For all positive integers$n$greater than$2$. Find the number of functions $$f:\{1,2,3, \ldots, n\} \rightarrow\{1,2,3,4,5\}$$ satisfying$|f(k+1)-f(k)| \geq 3$where$k \in\{1,2, \ldots, n-1\}$. ### Issue 411 1. The natural numbers$1,2 \ldots, 2011^{2}$are arranged in some order in a$2011 \times 2011$square table, each square contains one number. Prove that there exists two adjacent squares (that is two squares having a common edge or a common vertex) such that the difference between the corresponding assigned numbers is not smaller than$2012$. 2. Find the value of the following$2009$-terms sum $$S=\left(1+\frac{1}{1.3}\right)\left(1+\frac{1}{2.4}\right)\left(1+\frac{1}{3.5}\right) \ldots\left(1+\frac{1}{2009.2011}\right)$$ 3. Find the integers$x$,$y$satisfying the expression $$x^{3}+x^{2} y+x y^{2}+y^{3}=4\left(x^{2}+y^{2}+x y+3\right)$$ 4.$M$is a point in the interior of a triangle$A B C .$Let$P$,$Q$,$R$,$H$,$G$be respectively the centroid of triangles$M B C$,$M A C$,$M A B$,$P Q R$,$A B C$. Prove that points$M$,$H$and$G$are colinear. 5.$a$,$b$and$c$are positive real numbers whose sum is$3$. Prove the inequality $$\frac{4}{(a+b)^{3}}+\frac{4}{(b+c)^{3}}+\frac{4}{(c+a)^{3}} \geq \frac{a}{b+c}+\frac{b}{c+a}+\frac{c}{a+b}$$ 6. The incircle$(I)$of a triangle$A B C$touches$B C$,$C A$,$A B$at$D$,$E$,$F$respectively. The line passing through$A$and parallel to$B C$meets$E F$at$K$.$M$is the midpoint of$B C$. Prove that$I M$is perpendicular to$D K$. 7. Solve the system of equations $$\begin{cases}\sqrt{\dfrac{x^{2}+y^{2}}{2}}+\sqrt{\dfrac{x^{2}+x y+y^{2}}{3}}&=x+y \\ x \sqrt{2 x y+5 x+3} &=4 x y-5 x-3\end{cases}$$ 8. Let$a, b, c$be real numbers such that the equation$a x^{2}+b x+c=0$has two real solutions, both are in the closed interval$[0 ; 1] .$Find the maximum and mininum values of the expression $$M=\frac{(a-b)(2 a-c)}{a(a-b+c)}.$$ 9. Let$P(x)$and$Q(x)$be two polynomials with real coefficients, each has at least one real solution, so that $$P\left(1+x+Q(x)+(Q(x))^{2}\right)=Q\left(1+x+P(x)+(P(x))^{2}\right).$$ For any$x \in \mathbb{R}$. Prove that$P(x) \equiv Q(x)$. 10. Let$a, b, c, d$be positive numbers such that$a \geq b \geq c \geq d$and$a b c d=1 .$Find the smallest constant$k$such that the following inequality holds $$\frac{1}{a+1}+\frac{1}{b+1}+\frac{1}{c+1}+\frac{k}{d+1} \geq \frac{3+k}{2}.$$ 11. Find all continuous functions$f: \mathbb{R} \rightarrow \mathbb{R}$satisfying $$\{f(x+y)\}=\{f(x)+f(y)\}$$ for every$x, y \in \mathbb{R}$($[t]$is the largest integer not exceed$t$and$\{t\}=t-[t]$.) 12. Let$A B C$be a triangle,$P$is an arbitrary point inside the triangle. Let$d_{a}$,$d_{b}$,$d_{c}$be respectively the distances from$P$to$B C$,$C A$,$A B$;$R_{a}$,$R_{b}$,$R_{c}$are the circumradii of triangles$P B C$,$P C A$,$P A B$respectively. Prove that $$\frac{\left(d_{a}+d_{b}+d_{c}\right)^{2}}{P A \cdot P B \cdot P C} \geq \frac{\sqrt{3}}{2}\left(\frac{\sin A}{R_{a}}+\frac{\sin B}{R_{b}}+\frac{\sin C}{R_{c}}\right)$$ ### Issue 412 1. Pick$n$numbers$(n \geq 2)$from the first hundred natural numbers (from$1$to$100$) so that the sum of any two distinct numbers is a multiple of$6 .$What is the largest possible number$n$so that this can be done? 2. Given$A=\dfrac{5^{a}}{5^{b+c}}$and$B=\dfrac{5^{a}+2011}{5^{b+c}+2011}$where$a, b, c$are the side lengths of a triangle. Compare$A$and$B$. 3. Do there exists three integers$x$,$y$and$z$such that $$|x-2005 y|+|y-2007 z|+|z-2009 x|=2011^{x}+2013^{y}+2015^{z} ?$$ 4. Determine the following sum of$2011$terms $$S=\frac{1}{1^{4}+1^{2}+1}+\ldots+\frac{2011}{2011^{4}+2011^{2}+1}$$ 5. Given a circle$(O),$a chord$B C$($B C$is not a diameter) and point$A$moving on the major arc$B C$. Draw a circle$\left(O_{1}\right)$passing through$B$and touches$A C$at$A$, another circle$\left(O_{2}\right)$passing through$C$and touches$A B$at$A .\left(O_{1}\right)$meets$\left(O_{2}\right)$at a second point$D$, different from$A$. Prove that line$A D$always passes through a fixed point. 6. A quadrilateral$A B C D$with$A C \perp B D$is inscribed in a fixed circle$(O ; R)$. Let$p$be the perimeter of$A B C D$. Prove that $$\frac{A B^{2}}{p-A B}+\frac{B C^{2}}{p-B C}+\frac{C D^{2}}{p-C D}+\frac{D A^{2}}{p-D A} \geq \frac{4 R \sqrt{2}}{3}$$ 7. Solve the system of equations $$\begin{cases}(17-3 x) \sqrt{5-x}+(3 y-14) \sqrt{4-y} &=0 \\ 2 \sqrt{2 x+y+5}+3 \sqrt{3 x+2 y+11} &=x^{2}+6 x+13\end{cases}$$ 8. Prove that the following inequality holds for any triangles$A B C$$$\cos ^{2} \frac{A-B}{2}+\cos ^{2} \frac{B-C}{2}+\cos ^{2} \frac{C-A}{2} \geq 24 \sin \frac{A}{2} \sin \frac{B}{2} \sin \frac{C}{2}.$$ 9. Two circles$\left(C_{1}\right),\left(C_{2}\right)$are given such that the center$O$of$\left(C_{2}\right)$lies on$\left(C_{1}\right) .$Let$C$,$D$be their intersection points. Points$A$and$B$on$\left(C_{1}\right)$and$\left(C_{2}\right)$respectively such that$A C$touches$\left(C_{2}\right)$at$C$and$B C$touches$\left(C_{1}\right)$at$C$. The line$A B$intersects$\left(C_{2}\right)$at$E$and$\left(C_{1}\right)$at$F$.$C E$meets$\left(C_{1}\right)$at$G, C F$meets$G D$at$H$. Prove that$G O$intersects$E H$at the circumcenter of triangle$D E F$. 10. Let$a_{1}, a_{2} \ldots, a_{n}$be$n$positive real numbers such that $$a_{1}^{2}+a_{2}^{2}+\ldots+a_{k}^{2} \leq \frac{k(2 k-1)(2 k+1)}{3}$$ where any$k=\overline{1, n}$. Find the largest possible value of the expression $$P=a_{1}+2 a_{2}+\ldots+n a_{n}.$$ 11. Given a sequence$\left(x_{n}\right)$such that $$x_{n}=2 n+a \sqrt[3]{8 n^{3}+1}, \forall n=1,2, \ldots$$ where$a$is any real number. a) For what values of$a$does the sequence has finite limit? b) Find$a$such that$\left(x_{n}\right)$is eventually increasing. 12. Find all function$f: \mathbb{R} \rightarrow \mathbb{R}$such that $$f\left(x^{3}+y^{3}\right)=x^{2} f(x)+y^{2} f(y)$$ where$x, y \in \mathbb{R}$### Issue 413 1. On the cardboards, Write each five-digit numbers, from$11111$to$99999$, on a cardboard. After mixing the cardboards, place them in a sequence in certain order. Prove that the resulting number is not a power of$2 .$2. Find all triple of pairwise distinct prime numbers$a, b, c$such that $$20 a b c<30(a b+b c+c a)<21 a b c.$$ 3. Point$O$on the median$A D$of triangle$A B C$is chosen such that$\dfrac{A O}{A D}=k(0<k<1)$. The rays$B O$,$C O$cut$A C$,$A B$at$E$,$F$respectively. Determine the value of$k$so that $$S_{A E O F}=\frac{1}{15} S_{A B C}.$$ 4. Solve the equation $$\sqrt{x^{2}+x+19}+\sqrt{7 x^{2}+22 x+28}+\sqrt{13 x^{2}+43 x+37}=3 \sqrt{3}(x+3).$$ 5. Let$A B C$be a right triangle with right angle at$A$.$D$is a point within the triangle so that$C D=C A$. Choose point$M$on the edge$A B$so that$\widehat{B D M}=\dfrac{1}{2} \widehat{A C D}$;$N$is the intersection of$M D$and the altitude$A H$of triangle$A B C$. Prove that$D M=D N$. 6. Let$A B C$be a triangle inscribed the circle$(O)$.$F$is an arbitrary point on the arc$\widehat{A B}$(not containing$C$)$(F$differs from$A$and$B$).$M$is the midpoint of the arc$\widehat{B C}$(not containing$A$);$N$is the midpoint of the arc$\widehat{A C}$(not containing$B$). The line passing through$C$and parallel to$M N$cuts the circle$(O)$at another point$P .$Let$I$,$I_{1}$,$I_{2}$be the incenters of triangles$A B C$,$F A C$,$F B C$.$P I$cuts the circle$(O)$at$G$. Prove that the four points$I_{1}$,$F$,$G$,$I_{2}$are concyclic. 7. Solve the equation $$(2 \sin x-3)\left(4 \sin ^{2} x-6 \sin x+3\right)=1+3 \sqrt[3]{6 \sin x-4}.$$ 8.$x, y, z,$and$t$are four real numbers longing to the interval$\left[\frac{1}{2} ; \frac{2}{3}\right]$. Find the least and greatest values of the expression $$P=9\left(\frac{x+z}{x+t}\right)^{2}+16\left(\frac{x+t}{x+y}\right)^{2}$$ 9. Prove that given any prime number$p,$there exist natural numbers$x$,$y$,$z$,$t$so that$x^{2}+y^{2}+z^{2}-t p=0$and$0<t<p$. 10. Let$\left(u_{n}\right)$be a sequence given by $$u_{1}=a ,\quad u_{n+1}=\frac{(\sqrt{2}+1) u_{n}-1}{\sqrt{2}+1+u_{n}},\,\forall n \geq 1.$$ a) Find the condition of$a$so that all terms in the sequence are well-defined. b) Find the value of$a$such that$u_{2011}=2011$. 11. Find all functions$f: \mathbb{N}^{*} \rightarrow \mathbb{N}^{*}$satisfying the following condition $$\frac{f(x+y)+f(x)}{2 x+f(y)}=\frac{2 y+f(x)}{f(x+y)+f(y)}, \forall x, y \in \mathbb{N}^{*}$$ 12. Let$A B C$be a triangle,$\widehat{B A C} \neq 90^{\circ}$.$D$is a fixed point on the edge$B C$.$P$is a point inside the triangle$A B C$. Let$B_{1}$,$C_{1}$be respectively the projections of$P$onto$A C$,$A B$.$D B_{1}$cuts$A B$at$C_{2}$,$D C_{1}$cut$A C$at$B_{2}$.$Q$is the intersection differs from$A$of the circumcircles of triangles$A B_{1} C_{1}$and$A B_{2} C_{2}$. Prove that the line$P Q$always go through a fixed point when$P$is moving. ### Issue 414 1. Do there exist two natural numbers$a, b$such that $$(3 a+2 b)(7 a+3 b)-4=\overline{22} * 12 * 2011 ?$$ 2. Equilateral triangles$A B E$and$B C F$are constructed outside triangle$A B C .$Let$G$be the centroid of triangle$A B E$and$I$be the midpoint of$A C .$Find the measure of angle$G I F$. 3. Find the smallest positive integer$n$such that$2^{n}-1$is divisible by$2011$. 4. Prove that for all integers$k$, the equation $$x^{4}-2010 x^{3}+(2009+k) x^{2}-2007 x+k=0$$ does not have two distinct integer roots. 5. From a point$M$outside the cycle$(O),$draw the tangents$M A, M B$and the secant$M C D$to$(O), C$lies between$M$and$D .A B$cuts$C D$at$N .$Prove that $$\frac{1}{M D}+\frac{1}{N D}=\frac{2}{C D}$$ 6. Let$A B C$be a right triangle, right angle at$A,$satisfying$A B+\sqrt{3} A C=2 B C$. Find the position of point$M$such that $$4 \sqrt{3} \cdot M A+3 \sqrt{7} \cdot M B+\sqrt{39} \cdot M C$$ is smallest possible. 7. Solve the equation $$\log _{3}\left(7^{x}+2\right)=\log _{5}\left(6^{x}+19\right)$$ 8. Let$A B C$be a triangle satisfying $$\tan \frac{A}{2} \tan \frac{B}{2}=\frac{1}{2}.$$ Prove that$A B C$is a right triangle iff $$\sin \frac{A}{2} \sin \frac{B}{2} \sin \frac{C}{2}=\frac{1}{10}.$$ 9. Let$A B C$be a triangle inscribed the circle$(O ; R), M$is a point not on the circle respectively. Let$r$,$r_{1}$be respectively the radii of the incircles of triangles$A B C$and$A_{1} B_{1} C_{1}$. Prove that $$\left|R^{2}-O M^{2}\right| \geq 4 r \cdot r_{1}$$ 10. Find the greatest positive constant$k$satisfying the inequality $$\frac{k}{a^{3}+b^{3}}+\frac{1}{a^{3}}+\frac{1}{b^{3}} \geq \frac{16+4 k}{(a+b)^{3}}.$$ 11. Find all functions$f: \mathbb{R} \rightarrow \mathbb{R}$such that $$f(x f(y)+y)+f(x y+x)=f(x+y)+2 x y.$$ 12. For each positive integer$n$consider a function$f_{n}$in$\mathbb{R}$defined by $$f_{n}(x)=\sum_{i=1}^{2 n} x^{i}+1.$$ Prove the following statements a)$f_{n}$obtains its minimum value at a unique point$x_{n},$for each positive integer$n .$Put$S_{n}=f_{n}\left(x_{n}\right)$. b)$S_{n}>\dfrac{1}{2}$for all$n \in \mathbb{N}^{*} .$Moreover,$\dfrac{1}{2}$is the best constant possible in the sense that there does not exist any real number$a>\dfrac{1}{2}$such that$S_{n}>a$for all$n \in \mathbb{N}^{*}$. c) The sequence$\left(S_{n}\right)(n=1,2, \ldots)$is decreasing and$\displaystyle\lim_{n\to\infty} S_{n}=\dfrac{1}{2}$. d)$\displaystyle\lim_{n\to\infty} x_{n}=-1$. ##$hide=mobile$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 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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 2011
Mathematics and Youth Magazine Problems 2011