Good morning. I have this problem : << Two circles C and C' intersect at points A and B. A straight line passing through B intersects the circles C and C' respectively at M and N, with B between M and N. Let I be the midpoint of the arc AM of circle C not containing B, and J the midpoint of the arc AN of circle C' not containing B. If K is the middle of MN, prove that KI is perpendicular to KJ. >> Thank you for any help.