(1 -cos a/2)*(1+cos a/2)

Question

(1 -cos a/2)*(1+cos a/2)

Answer 1

...=1 - cos^2(a/2) = sin^2(a/2).
1-2*sin^2(a/2) = cos(a),
sin^2(a/2) = (1-cos(a))/2