y=√(x-1)+1(x≥1)
所以 y>=1
y-1=√(x-1)(x≥1,y>=1)
x - 1 = (y-1)^2 = y^2 - 2y + 1
x = y^2 - 2y + 2
反函数是 y' = x^2 - 2x + 2 (x≥1)