f（1-cosx）=sin²x
=1-cos²x
=-cos²x+2cosx-2cosx+1
=-cos²x+2cosx-1-2cosx+2
=-(1-cosx)²+2(1-cosx)
f(x)=-x²+2x
f(x²)=-(x²)²+2x²
=-x^4+2x²