Replace cos(B/2) with cos(A/2) and cos(D/2) with cos(C/2). Then replace cos(C/2) with cos((π - A)/2).
I've been racking my head about trying to do the following problem, but am unable to solve it:
"Quadrilateral ABCD is an isosceles trapezoid with AB parallel to CD. Find the value of P = cos(A/2) * cos(B/2) + cos(C/2) * cos(D/2)"
I started off with A+D = 180, and tried taking the sin and cos of both sides, but not sure where to continue after that.