f ' (sinx）=sin3x
=sin(x+2x)
=sinxcos2x+cosxsin(2x)
=sinx(1-2sin^2x)+cosx*2sinxcosx
f'(x)=x(1-2x^2)+2x(1-x^2)
=3x-4x^3=sin(x+2x)=sinxcos2x+cosxsin(2x)怎么来的？什么定理？三角函数两角和公式呀