The points $A, B, C, D$ lie in that order on a circle. The segments $AC$ and $BD$ intersect at the point $P$. The point $B'$ lies on the line $AB$ such that $A$ is between $B$ and $B'$ and $|AB'| = |DP |$. The point $C'$ lies on the line $CD$ such that $D$ is between $C$ and $C'$ lies and $|DC' | = |AP|$. Prove that $\angle B'PC' = \angle ABD'$. =

A domino is a rectangle whose length is twice its width. Any square can be divided into seven dominoes, for example as shown in the figure below. a) Show that you can divide a square into $n$ dominoes for all $n \ge 5$. b) Show that you cannot divide a square into three or four dominoes.

Arne has $2n + 1$ tickets. Each card has one number on it. One card has the number $0$ on it. The natural numbers $1, 2, . . . , n$ occur on exactly two cards each. Prove that Arne can arrange cards in a row so that there are exactly $m$ cards between the two cards with the number $m$, for every $m \in \{1, 2, . . . , n\}$.

Determine all real polynomials $P$ of degree at most $22$ for which $$kP (k + 1) - (k + 1)P (k) = k^2 + k + 1$$for all $k \in \{1, 2, 3, . . . , 21, 22\}$.