Let A B C D be a trapezoid such that A B \| C D and let P=A C \cap B D, A B=21, C D=7, A D=13,[A B C D]=168. Let the line parallel to A B trough P intersect circumcircle of B C P in X. Circumcircles of B C P and A P D intersect at P, Y. Let X Y \cap B C=Z. If \angle A D C is obtuse, then B Z=\frac{a}{b}, where a, b are coprime positive integers. Compute a+b.