(1 - cos^2 α) = sin^2 α(1 + cos α) = 2cos^2(α/2)
Поэтому (1 - cos α)(1 + cos α) = sin^2 α 2cos^2(α/2) = 2sin^2 α cos^2(α/2)
(1 - cos^2 α) = sin^2 α
(1 + cos α) = 2cos^2(α/2)
Поэтому (1 - cos α)(1 + cos α) = sin^2 α 2cos^2(α/2) = 2sin^2 α cos^2(α/2)