The function is
,
.
The function
is continuous over closed interval
, and differentiable on the open interval
.
Therefore mean value theorem can be applied.
\So, there exists at least one number
in
such that
.
The slope of the secant line through
and
is

.
The function is
.
Apply the derivative on each side with respect to
.

Result of mean value theorem : 
.
.