1 - tan^2x cos^2x/cos^2x - sin^2x/cos^2x [ cos^2x - sin^2x ] * cosx
________ = ____________________________ = _____________________ =
2 tan x 2 sinx / cosx 2sinx *cos^2 x
cos^2x - sin^2x cos 2x
____________ = _________ = cot2x = 1 / tan2x ⇒ Identity
2sinxcosx sin2x