f(sinx)=sin2x-sin3x 则f（cosx）=

f(sinx)=2sinxcosx-sin2xcosx-cos2xsinx
=2sinx(1-sinx^2)^(1/2)-2sinx(1-sinx^2)-(1-2sinx^2)*sinx
所以f(x)=2x(1-x^2)^(1/2)-2x(1-x^2)-(1-2x^2)x
f(cosx)=2cosx(1-cosx^2）^(1/2)-2cosx(1-cosx^2)-(1-2cosx^2)cosx
=2cosxsinx-2cosxsinx^2+(2cosx^2-1)cosx
=sin2x-sin2xsinx+cos2xcosx
=sin2x+cos3x