2sinxcosx=tanx??
2sinxcx=tanx??
tanx=sinx/cx补充：(tanx)'=（sinx/cosx）'=(sin'a cosa-cos'a sina)/cosa^2sin(x+x)=sinxcosx+cosxsinx=2sinxcosx cos(x+x)=cosxcosx-sinxsinx=(cosx)^2-(sinx)^2 另外两个分别加和减1=(sinx)^2+(cosx)^2 sin2x=2sinxcos根据sin(a+b)=sinacosb+cosasinb推出 cos2x=1-(2sinx)^2 or (2cosx)^2-1,同理用 cos(a+b)=cosacosb-sinasinb推导 sin2x =sin(x+x) =sinxcosx+cosxsinx =2sinxcosx cos2x =cos(x+x) =cosxcosx-sinxsinx =cos^2x-sin^2x tan2x =tan(x+x) =(tanx+tanx)/(1-tanxtanx) =2tanx/(1-tan^2x) =(cosa^2+sina^2)/cosa^2=1/cosa^2