换元法f（1-cosx)=sin^2 x

f（1-cosx)
=sin^2 x
=1-cos^2x
=(1+cosx)(1-c0sx)
=-(1-cosx-2)(1-cosx)
所以
f(x)=-(x-2)x
=-x^2+2x

1-cosx=t,cosx=1-t
sin^2x=1-cosx^2
=1-(1-t)^2
=-t^2+2t
f(x)=-x^2+2x

令t=1-cosx 则cosx=1-t
所以f(t)=f(1-cosx)=sin^2x=1-cos^2x=1-(1-t)^2=-t^2+2t
x、t只是表示一种符号，将t换成x
则f(x)=-x^2+2x

令1-cosx=y,cosx=1-y
f(y)=1-cos^2x=1-(1-y)^2=-y^2+2y

f(x)=-x*x+2X