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2015 Issue 459

  1. Let $$\begin{align*} A & =\frac{1}{1.2^{2}}+\frac{2}{2.3^{2}}+\frac{1}{3.4^{2}}+\ldots+\frac{1}{49.50^{2}},\\ B & =\frac{1}{2^{2}}+\frac{1}{3^{2}}+\frac{1}{4^{2}}+\ldots+\frac{1}{50^{2}}.\end{align*}$$ Compare $A$ and $B$ with $\dfrac{1}{2}$.
  2. For all pairs of real numbers $(a,b)$ such that the polynomial \[A(x)=x^{2}-2ax+2a^{2}+b^{2}-5\] has solutions. Find the minimum value of the expression \[P=(a+1)(b+1).\]
  3. Suppose that $a_{1},a_{2},\ldots,a_{n}$ are different positive integers such that \[\frac{1}{a_{1}}+\frac{1}{a_{2}}+\ldots+\frac{1}{a_{n}}=1\] and also assume that the biggest number among them is equal to $2p$, where $p$ is some prime number. Determine $a_{i}$ ($i=\overline{1,n}$).
  4. Let $ABC$ be a isosceles right triangle ($AB=BC$) and let $O$ be the midpoint of $AC$. Through $C$ draw the line $d$ perpendicular to $BC$. Let $Cx$ be the opposite ray of the ray $CB$ and $M$ be an arbitrary point on $Cx$. Assume that $E$ is the intersection between $BE$ and $OM$. Prove that when $M$ varies on $Cx$, $I$ always belongs to a fixed curve.
  5. Solve the following equation \[x^{2}-2x=2\sqrt{2x-1}.\]
  6. Solve the following inequation \[\frac{2x^{3}+3x}{7-2x}>\sqrt{2-x}.\]
  7. Given an acute and non-isosceles triangle $ABC$ with the altitudes $AH$, $BE$, $CF$. Let $I$ be the incenter of $ABC$ (the center of the inscribed circle) and $R$ be the circumradius of $ABC$ (the radius of the circumscribed circle). Let $M,N$ and $P$ respectively be the midpoint of $BC,CA$ and $AB$. Assume that $K,J$ and $L$ respectively are the intersections between $MI$ and $AH$, $NI$ and $BE$, and $PI$ and $CF$. Prove that \[\frac{1}{HK}+\frac{1}{EJ}+\frac{1}{FL}>\frac{3}{R}.\]
  8. Let $a,b,c$ be the lengths of three sides of a triangle whose perimeter is equal to $3$. Find the minimum value of the expression \[T=a^{3}+b^{3}+c^{3}+\sqrt{5}abc.\]
  9. For every positive integer $n$ prove that the following numbers are perfect square \[ 10([(4+\sqrt{15})^{n}+1])([(4+\sqrt{15})^{n+1}]+1)-60,\] \[6([(4+\sqrt{15})^{n}+1])([(4+\sqrt{15})^{n+1}]+1)-60,\] \[15([(4+\sqrt{15})^{n}+1])^{2}-60,\] where $[x]$ is the integerpart of $x$.
  10. Let $f(x)=x^{3}-3x^{2}+1$.
    a) Find the number of different real solutions of the equation $f(f(x))=0$.
    b) Let $\alpha be$the maximal positive solution of $f(x)$.
    Prove that $[\alpha^{2020}]$ is divisible by $17$ (notice that $[x]$ is the integer part of $x$).
  11. Give the sequence $(u_{n})$ where $u_{1}=2$, $u_{2}=20$, $u_{3}=56$ and \[u_{n+3}=7u_{n+2}-11u_{n+1}+5u_{n}-3.2^{n},\quad\forall n\in\mathbb{N}^{*}.\] Find the remainder of the division $u_{2011}$ by $2011$.
  12. Consider any triangle $ABC$ and any line $d$. Let $A_{1},B_{1},C_{1}$ are the projections of $A,B,C$ onto $d$. It is a fact that the line through $A_{1}$ and perpendicular to $BC$, the line through $B_{1}$ and perpendicular to $AC$, and the line through $C_{1}$ and perpendicular to $AB$ are concurrent ar a point which is called the orthogonal pole of $d$ with respect to $ABC$. Prove that for any triangle and any point $P$ on its circumcircle, the Simon line corresponding to $P$ and the orthogonal pole of the Simon line with respect to the given triangle.

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Mathematics & Youth: 2015 Issue 459
2015 Issue 459
Mathematics & Youth
https://www.molympiad.org/2017/08/mathematics-and-youth-magazine-problems_86.html
https://www.molympiad.org/
https://www.molympiad.org/
https://www.molympiad.org/2017/08/mathematics-and-youth-magazine-problems_86.html
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