Points A,B,C,D are on circle S, such that AB is the diameter of S, but CD is not the diameter. Given also that C and D are on different sides of AB. The tangents of S at C and D intersect at P. Points Q and R are the intersections of line AC with line BD and line AD with line BC, respectively.
(a) Prove that P, Q, and R are collinear.
(b) Prove that QR is perpendicular to line AB. geometrycircumcirclegeometry proposed