formula for calculating the area of a segment is as follows;
A = ½ x r^2(ϴ - sin(ϴ)
where r is the radius of the circle and ,ϴ, is the angle in degrees of the sector that contains a segment.
If it's only the A of a segment it's calculated as follows
A = ½ × r^2 × ((π/180) ϴ - sin ϴ)