2009 AIME II Problems/Problem 15

Let be a diameter of a circle with diameter 1. Let A and B be points on one of the semicircular arcs determined by MN such that A is the midpoint of the semicircle and MB=3/5. Point C lies on the other semicircular arc. Let d be the length of the line segment whose endpoints are the intersections of diameter MN with chords AC and BC. The largest possible value of d can be written in the form r-s*sqrt (t), where r, s, and t are positive integers and t is not divisible by the square of any prime. Find r+s+t.