令y=2x-1,则x=(y+1)/2,代入f(2x-1) = x^2+4x可得
f(y) = (y+1)^2/4 + 4 * (y+1) / 2
= (y^2+2y+1) / 4 + 2y + 2
= y^2/4 + 5y/2 + 9/4
所以
f(x) = x^2/4 + 5x/2 + 9/4