y = f(x)
反函數(shù)：x = g(y)
g() = f'() ---- f 的反函數(shù)
dg(x)/dx = dx/dy = 1/(dy/dx) = 1/[df(x)/dx]
即："反函數(shù)的導(dǎo)數(shù)是原函數(shù)的導(dǎo)數(shù)的倒數(shù)"
舉例：原函數(shù)：y ＝ ln x
反函數(shù)：x ＝ e^(y)
即：反函數(shù)x =e^(y)的導(dǎo)數(shù)dx/dy=e^(y)等于原函數(shù)y=ln x的導(dǎo)數(shù)(1/x)的倒數(shù)x,即e^(y).