Question 17
A center circumference $O$ passes through the vertices $A$ and $C$ of a triangle $A B C$ and cuts the segments again $A B$ and $B C$ at different points $K$ and $N$, respectively. Circumferences circumscribed to triangles $A B C$ and $K B N$ are cut exactly at two different points $B$ and $M$. Prove that the angle $\widehat{O M B}$ is a right angle.