sin²α + 1 - 2sin²α /(1 - cos²α) = sin²α + 1 - 2sin²α /sin²α =
= sin²α + 1 - 2 = -(1 - sin²α) = -cos²α
sin²α + 1 - 2sin²α /(1 - cos²α) = sin²α + 1 - 2sin²α /sin²α =
= sin²α + 1 - 2 = -(1 - sin²α) = -cos²α