f(1-sinx)=cos²x
=1-sin²x
=(1+sinx)(1-sinx)
=[2-(1-sinx)] (1-sinx)
∴f(x)=(2-x)x
=2x-x²
=-(x²-2x+1)+1
=-(x-1)²+1
函数定义域为R
函数值域为：(-∞,1]