The area of a sector of a circle is given by:
A = (1 / 2) r2 θ
where θ is measured in radians. Additionally, the arc length of a segment of a circle, s, is related to the radius of the circle by:
s = r θ -> θ = s / r
A = (1 / 2) r2 (s / r) -> A = (1 / 2) r a
Finally, because s = a, and r = a:
A = (1 / 2) * a2