The Position function is
and the time interval is
.
The velocity is defined as rate of change of position with respect to time
in the interval
is
.
The velocity in the interval
:

Average velocity is
.
Find instantaneous velocity at
.
Instantaneous velocity is the derivative of the position function with respect to
.
Consider the Position function is
.
Apply derivative on each side with respect to
.

Substitute
in the above function.
Instantaneous velocity
.
Hence the average velocity in the interval
is same as instantaneous velocity at
.
Average velocity is
.
Instantaneous velocity
.
The average velocity in the interval
is same as instantaneous velocity at
.