1-cos ^2x=sin^2x 方程两边同时乘以sinx^2/cosx^2
sin^2x/cos^2x - sin^2x =sin^2x sin^2x/cos^2x
tan^2x -sin^2x =sin^2 x tan^2 x
(tanx-sinx)(tanx+sinx)=(sinx tanx) (sinx tanx)
divide both sides by sinxtanx/(tanx+sinx)
tanx-sinx/tanx sinx=tanxsinx/tanx+sinx
tanxsinx/tanx+sinx = tanx-sinx/tanx sinx
所以,相等
tanx.sinx/tanx-sinx=tanx+sinx/tanxsinx