Semiperimeter
From AoPSWiki
The semiperimeter of a geometric figure is one half of the perimeter, or 
, where 
is the total perimeter of a figure. It is typically denoted 
.
Applications
The semiperimeter has many uses in geometric formulas. Perhaps the simplest is 
, where 
is the area of a triangle and 
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.
