Question

ABCD is a cyclic quadrilateral find value of cosA.cosB - cos C.cosD.

Answer 1

$\cos(a) = \cos(180 - c) \\ \bold{ \cos(a) = - \cos(c )} \\ similarly \\ \bold{ \cos(b) = - \cos(d) } \\ \\ \\ \cos(a) \cos(b) - \cos(c) \cos(d) \\ = \cos(a) \cos(b) - \cos(a) \cos(b) \\ = 0$
please follow me