令a=2x-1
x=(a+1)/2
f(a)=(a+1)/2-(a+1)²/4=(-a²+1)/4
f(x)=(-x²+1)/4
f(2x-1)=x-(x的平方)
=-(x^2-x)
=-1/4(2x-1)^2+1/4
所以 f(x)=-x^2/4+1/4