（1）sin^4α-cos^4α=2sin^2α-1 （2）tan^2α-sin^2α=tan^2αsin^2α （3）cosα/1-sinα=1+sinα/cos（1）sin^4α-cos^4α=2sin^2α-1 （2）tan^2α-sin^2α=tan^2αsin^2α （3）cosα/1-sinα=1+sinα/cosα求证
问题描述：
（1）sin^4α-cos^4α=2sin^2α-1 （2）tan^2α-sin^2α=tan^2αsin^2α （3）cosα/1-sinα=1+sinα/cos
（1）sin^4α-cos^4α=2sin^2α-1
（2）tan^2α-sin^2α=tan^2αsin^2α
（3）cosα/1-sinα=1+sinα/cosα
求证
答
(1) 设 x= (sin α)^2, 则 (cos α)^2 = 1-x. 所以 (sin α)^4 - (cos α)^4 = x^2 - (1-x)^2 = 2x -1 = 2(sin α)^2 -1.(2) 设...