The beam deflection is
.
Where
is length of the beam.

Apply derivative with respect to
.

Find the relative extremas by equating the first derivative to zero.
\
Find the maximum deflection using second derivative test.
\
Apply derivative with respect to
.

Substitute
in
.
.
So the function has minimum value at
.
Substitute
in
.

So the function has maximum value at
.
The function has maximum value at
.