(cos a/8 - sin a/8)(cos a/8 + sin a/8)* 2 sin a/4 =
(cos² a/8 - sin² a/8)* 2 sin a/4 =
((1+cos a/4)/2 -(1-cos a/4)/2)* 2 sin a/4 =
cos a/4 * 2 * sin a/4 = sin a/2
(cos a/8 - sin a/8)(cos a/8 + sin a/8)* 2 sin a/4 =
(cos² a/8 - sin² a/8)* 2 sin a/4 =
((1+cos a/4)/2 -(1-cos a/4)/2)* 2 sin a/4 =
cos a/4 * 2 * sin a/4 = sin a/2