Let OAPB be a sector of a circle with centre O and radius r (see Fig. 11.3). Let the degree measure of ∠AOB be θ. You know that area of a circle (in fact of a circular region or disc) is πr². In a way, we can consider this circular region to be a sector forming an angle of 360° (i.e., of degree measure 360) at the centre O. Now by applying the Unitary Method, we can arrive at the area of the sector OAPB as follows: When degree measure of the angle at the centre is 360, area of the sector = πr². So, when the degree measure of the angle at the centre is 1, area of the sector = πr²/360. Therefore, when the degree measure of the angle at the centre is θ, area of the sector = (πr²/360) × θ = (θ/360) × πr². Thus, we obtain the following relation (or formula) for area of a sector of a circle: Area of the sector of angle θ = (θ/360) × πr², where r is the radius of the circle and θ the angle of the sector in degrees.