# Brahmagupta's Formula

**Brahmagupta's Formula** is a formula for determining the area of a cyclic quadrilateral given only the four side lengths.

## Definition

Given a cyclic quadrilateral with side lengths , , , , the area can be found as:

where is the semiperimeter of the quadrilateral.

### Proof

If we draw , we find that . Since , . Hence, . Multiplying by 2 and squaring, we get:

\[4[ABCD]}^2=\sin^2 B(ab+cd)^2\] (Error compiling LaTeX. ! Extra }, or forgotten $.)

Substituting results in By the Law of Cosines, . , so a little rearranging gives

## Similar formulas

Bretschneider's formula gives a formula for the area of a non-cyclic quadrilateral given only the side lengths; applying Ptolemy's Theorem to Bretschneider's formula reduces it to Brahmagupta's formula.

Brahmagupta's formula reduces to Heron's formula by setting the side length .

A similar formula which Brahmagupta derived for the area of a general quadrilateral is
where is the semiperimeter of the quadrilateral. What happens when the quadrilateral is cyclic?
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