IMO 1998

Problem 1. In the convex quadrilateral $ABCD$, the diagonals $AC$ and $BD$ are perpendicular and the opposite sides $AB$ and $DC$ are not parallel. Suppose that the point $P$, where the perpendicular bisectors of $AB$ and $DC$ meet, is inside $ABCD$. Prove that $ABCD$ is a cyclic quadrilateral if and only if the triangles $ABP$ and $CDP$ have equal areas.

Problem 2. In a competition, there are $a$ contestants and $b$ judges, where $b\ge 3$ is an odd integer. Each judge rates each contestant as either "pass" or "fail". Suppose $k$ is a number such that, for any two judges, their ratings coincide for at most $k$ contestants. Prove that $k/a\ge (b-1)/(2b)$.

Problem 3. For any positive integer $n$, let $d(n)$ denote the number of positive divisors of $n$ (including 1 and $n$ itself). Determine all positive integers $k$ such that $d(n^2)/d(n)=k$ for some $n$.

Problem 4. Determine all pairs $(a,b)$ of positive integers such that $a{b}^2+b+7$ divides $a^{2}b+a+b$.

Problem 5. Let $I$ be the incenter of triangle $ABC$. Let the incircle of $ABC$ touch the sides $BC$, $CA$, and $AB$ at $K$, $L$, and $M$, respectively. The line through $B$ parallel to $MK$ meets the lines $LM$ and $LK$ at $R$ and $S$, respectively. Prove that angle $RIS$ is acute.

Problem 6. Consider all functions $f$ from the set $N$ of all positive integers into itself satisfying $f(t^{2}f(s))=s(f(t){)}^2$ for all $s$ and $t$ in $N$. Determine the least possible value of $f(1998)$.