the limit of a composite function and the arithmetic properties of the limit,
we find
lim r1{y)-rVQ)= Jim x-xo _
YBy-^yo y-VO XBx-^xo f(x) - f(x0)
1 1
= lim
XBx-^xo (/(*)-/(*(>)) f'(x0) '
X XQ /
Thus we have shown that the function / _ 1 : Y -» X has a derivative at
?/o and that
(f-'YiVo) = (fixo))-1 • D