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\hfill \thepage} %} \input{tcilatex} \begin{document} \section{Twenty-fifth International Olympiad, 1984} 1984/1. Prove that $0\le yz+zx+xy-2xyz\le 7/27,$ where $x,y$ and $z$ are non-negative real numbers for which $x+y+z=1.$ 1984/2. Find one pair of positive integers $a$ and $b$ such that: (i) $ab(a+b)$ is not divisible by $7;$ (ii) $(a+b)^{7}-a^{7}-b^{7}$ is divisible by $7^{7}$ . Justify your answer. 1984/3. In the plane two different points $O$ and $A$ are given. For each point $X$ of the plane, other than $O$, denote by $a(X)$ the measure of the angle between $OA$ and $OX$ in radians, counterclockwise from $OA(0\le a(X)<2\pi ).$ Let $C(X)$ be the circle with center $O$ and radius of length $% OX+a(X)/OX.$ Each point of the plane is colored by one of a finite number of colors. Prove that there exists a point $Y$ for which $a(Y)>0$ such that its color appears on the circumference of the circle $C(Y).$ 1984/4. Let $ABCD$ be a convex quadrilateral such that the line $CD$ is a tangent to the circle on $AB$ as diameter. Prove that the line $AB$ is a tangent to the circle on $CD$ as diameter if and only if the lines $BC$ and $% AD$ are parallel. 1984/5. Let $d$ be the sum of the lengths of all the diagonals of a plane convex polygon with $n$ vertices $(n>3)$, and let $p$ be its perimeter. Prove that $n-3<\frac{2d}{p}<\left[ \frac{n}{2}\right] \left[ \frac{n+1}{2}\right] -2,$ where $[x]$ denotes the greatest integer not exceeding $x.$ 1984/6. Let $a,b,c$ and $d$ be odd integers such that \$0