Find expressions for (sin t/2)2 and (cos t/2)2 from the Pythagorean theorem and the cosine addition theorem.
The Pythagorian theorem is the statement (sin x)2 + (cos x)2 = 1 (or we can interpret it to be that statement).
If we apply this equation for and also use the cosine addition theorem
cos (u + v) = (cos u)(cos v) - (sin u)(sin v)
for , we get
and we can add and subtract these equations from one another to get