f(x)=(sinx)^2
令g(x)=sinx
f(g(x))=g(x)^2
f'(g(x))=2*g(x)
g'(x)=cosx
[f(g(x))]'=f'(g(x))*g'(x)=2*g(x)*g'(x)=2*sinx*cosx=sin2x