In geometry, the semiperimeter of a polygon is half its perimeter Although it has such a simple derivation from the perimeter, the semiperimeter appears frequently enough in formulas for triangles and other figures that it is given a separate name. When the semiperimeter occurs as part of a formula, it is typically denoted by the letter s. The semiperimeter is used most often for triangles; the formula for the semiperimeter of a triangle with side lengths a, b, and c is: The simplest form of Brahmagupta's formula, for the area of a cyclic quadrilateral, has a similar form: