AT-OEMO-REG 2019

Let $x,y$ be real numbers such that $(x+1)(y+2)=8.$ Prove that $$(xy-10)^2\ge 64.$$

The convex pentagon $ABCDE$ is cyclic and $AB = BD$ . Let point $P$ be the intersection of the diagonals $AC$ and $BE$ . Let the straight lines $BC$ and $DE$ intersect at point $Q$ . Prove that the straight line $PQ$ is parallel to the diagonal $AD$ .

Let $n\ge 2$ be a natural number. An $n \times n$ grid is drawn on a blackboard and each field with one of the numbers $-1$ or $+1$ labeled. Then the $n$ row and also the $n$ column sums calculated and the sum $S_n$ of all these $2n$ sums determined. (a) Show that for no odd number $n$ there is a label with $S_n = 0$ . (b) Show that if $n$ is an even number, there are at least six different labels with $S_n = 0$ .

Find all natural numbers $n$ that are smaller than $128^{97}$ and have exactly $2019$ divisors.