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In geometry, a quadrilateral, quadrangle or tetragon is a polygon with four sides.

## Types

The following are special cases of the quadrilateral:

## Area

Brahmagupta's formula finds the area of any quadrilateral given the lengths of the sides is

$A= \sqrt{\frac{(a^2+b^2+c^2+d^2)^2+8abcd-2(a^4+b^4+c^4+d^4)}{16}}$