cos2x =(1-tan^2x)/(1+tan^2x) ,你写错了.
这公式叫做万能公式.
证明：cos2x = cos^2x - sin^2x
= (cos^2x - sin^2x)/ ( cos^2x +sin^2x) 注意：cos^2x +sin^2x =1
= (1-tan^2x)/(1+tan^2x) 分子、分母同除cos^2x,已知sinx=2cosx,则sin^2+1等于，那这道题怎么做？30分sinx = 2cosxtanx=2sin^2x +1 = sin^2 + (sin^2x +cos^2)=2sin^2x +cos^2 =(2sin^2x +cos^2 )/ (sin^2 +cos^2x)=( 2tan^2x +1) /( tan^2x +1)= 9/5上面讲的一样原方法。归纳：如果分子，分母能写成正弦和余弦的同次，那么一般用余弦除，得到正切