(1-cos2a)^2 / (1+cos2a)^2 = (2sin^2 a)^2 / (cos^2 a)^2 左边的等于右边我要过程···

(1-cos2a)^2 / (1+cos2a)^2
=[1-(1-2sin^2 a)]^2 / [1+(2cos^2 a)-1]^2
=[1-1+2sin^2 a]^2 / [1+2cos^2 a-1]^2
=(2sin^2 a)^2 / (2cos^2 a)^2
=(sin^2 a)^2 / (cos^2 a)^2