Problem 4 (due Monday, March 30 )
Let $n>0$ be an odd integer. Prove that there exists a set $S=\{A_1, \ldots, A_{2n}\}$ of $2n$ distinct points in the plane which are not collinear and such that if $i+j\neq 2n+1$ then the line $A_iA_j$ contains a third point from $S$.