A quadrilateral inscribed is a circle is known as a cyclic quadrilateral. The proof is beyond the scope of this tutorial and will discuss in advanced tutorial, only the formula is given for application.
If $a$, $b$, $c$ and $d$are the sides of a cyclic quadrilateral and if $s = \frac{{a + b + c + d}}{2}$, then

Area of cyclic quadrilateral $= \sqrt {(s - a)(s - b)(s - c)(s - d)}$

Example:

In a circular grassy plot, a quadrilateral shape with its corners touching the boundary of the plot is to be paved with bricks. Find the area of the quadrilateral when the sides of the quadrilateral are $36$m, $77$m, $75$m and $40$m.

Solution:

Given the sides of the quadrilateral are $a = 36$m, $b = 77$m, $c = 75$m and $d = 40$m

Area of cyclic quadrilateral $= \sqrt {(s - a)(s - b)(s - c)(s - d)}$
Area of cyclic quadrilateral $= \sqrt {(114 - 36)(114 - 77)(114 - 75)(114 - 40)}$
Area of cyclic quadrilateral $= \sqrt {78 \times 37 \times 39 \times 74} = 39 \times 37 \times 2 = 2886$ Square meter.