PAMO 2009 Problem 6

Points $C,E,D$ and $F$ lie on a circle with centre $O$ . Two chords $CD$ and $EF$ intersect at a point $N$ . The tangents at $C$ and $D$ intersect at $A$ , and the tangents at $E$ and $F$ intersect at $B$ . Prove that $ON\perp AB$ .