\hfill \thepage} %} \input{tcilatex} \begin{document} \section{Twenty-first International Olympiad, 1979} 1979/1. Let $p$ and $q$ be natural numbers such that \[ \frac{p}{q}=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\cdots -\frac{1}{1318}+% \frac{1}{1319}. \] Prove that $p$ is divisible by $1979.$ 1979/2. A prism with pentagons $A_{1}A_{2}A_{3}A_{4}A_{5}$ and $% B_{1}B_{2}B_{3}B_{4}B_{5}$ as top and bottom faces is given. Each side of the two pentagons and each of the line-segments $A_{i}B_{j}$ for all $% i,j=1,...,5,$ is colored either red or green. Every triangle whose vertices are vertices of the prism and whose sides have all been colored has two sides of a different color. Show that all $10$ sides of the top and bottom faces are the same color. 1979/3. Two circles in a plane intersect. Let $A$ be one of the points of intersection. Starting simultaneously from $A$ two points move with constant speeds, each point travelling along its own circle in the same sense. The two points return to A simultaneously after one revolution. Prove that there is a fixed point $P$ in the plane such that, at any time, the distances from $P$ to the moving points are equal. 1979/4. Given a plane $\pi ,$ a point $P$ in this plane and a point $Q$ not in $\pi ,$ find all points $R$ in $\pi $ such that the ratio $(QP+PA)/QR$ is a maximum. 1979/5. Find all real numbers a for which there exist non-negative real numbers $x_{1},x_{2},x_{3},x_{4},x_{5}$ satisfying the relations \[ \sum_{k=1}^{5}kx_{k}=a,\sum_{k=1}^{5}k^{3}x_{k}=a^{2},% \sum_{k=1}^{5}k^{5}x_{k}=a^{3}. \] 1979/6. Let $A$ and $E$ be opposite vertices of a regular octagon. A frog starts jumping at vertex $A$. From any vertex of the octagon except $E,$ it may jump to either of the two adjacent vertices. When it reaches vertex $E,$ the frog stops and stays there.. Let $a_{n}$ be the number of distinct paths of exactly $n$ jumps ending at $E.$ Prove that $a_{2n-1}=0,$% \[ a_{2n}=\frac{1}{\sqrt{2}}(x^{n-1}-y^{n-1}),n=1,2,3,\cdots , \] where $x=2+\sqrt{2}$ and $y=2-\sqrt{2}.$ Note. A path of $n$ jumps is a sequence of vertices $(P_{0},...,P_{n})$ such that (i) $P_{0}=A,P_{n}=E;$ (ii) for every $i,0\le i\le n-1,P_{i}$ is distinct from $E;$ (iii) for every $i,0\leq i\leq n-1,P_{i}$ and $P_{i+1}$ are adjacent. \end{document}