One half is the irreducible fraction resulting from dividing one by two (½), or any number by its double; multiplication by one half is equivalent to division by two. It is the fraction occurring most often in mathematical equations, recipes, measurements, etc. Half can also be said to be one part of something divided into two equal parts. For instance, the area S of a triangle is computed S = ½ × base × perpendicular height. One half also figures in the formula for calculating figurate numbers, such as triangular numbers and pentagonal numbers: ½ × n [LINK: (s] - 2) n - (4 - s) and in the formula for computing magic constants for magic squares M2(n) = ½ × [LINK: n] (