y = f(x)
g是f的反函数
如果y1 = f(x1),y2 = f(x2)
则x1 = g(y1),x2 = g(y2)
根据导数定义,g' = g(y1)-g(y2)/y1-y2 = x1-x2/f(x1)-f(x2) = 1/f