\
Isosceles triangle is inscribed in a circle.
\
be the radius of the circle.
be the base of the triangle.
Observe the figure.
\From the figure, height of the triangle is
.
From the pythagorean theorem,
\
The area of the triangle is
.

The area of the triangle is maximum when its first derivative becomes zero.
\Apply derivative on each side with respect to
.

\
Equate
to zero.

The general solution for a quadratic equation is
.

Here
can not benegative.
Hence
.
Substitute
in
.

Therefore the height of the triangle is
.
Substitute
in
.

Therefore the base of the triangle is
.
\
The height of the triangle is
.
The base of the triangle is
.