cos2 (x/2) = 1 + cos x
2
cos 2(x/2) = cos2 (x/2) - sin2 (x/2)
cos x = cos2(x/2) - (1 - cos2 (x/2))
cos x = 2cos2(x/2) - 1
cos x + 1 = 2cos2(x/2)
1 + cos x = cos2 (x/2)
2
cos2 (x/2) = 1 + cos x
2