Semiperimeter

The semiperimeter of a geometric figure is one half of the perimeter, or $\frac{P}{2}$, where $P$ is the total perimeter of a figure. It is typically denoted $s$.

Applications

The semiperimeter has many uses in geometric formulas. Perhaps the simplest is $A=rs$, where $A$ is the area of a triangle and $r$ is the triangle's inradius (that is, the radius of the circle inscribed in the triangle).

Two other well-known examples of formulas involving the semiperimeter are Heron's formula and Brahmagupta's formula.