Theorem 3. (The derivative of an inverse function). Let the functions f :
X -» Y and f~1:Y-^Xbe mutually inverse and continuous at points
XQ G X and /(#o) — Vo £ Y respectively. If f is differentiable at xo and
f'(xo) 7^ 0, then f~l is also differentiable at the point y0, and
{f-'YiVo) = (f'ixo))-1 .