We have three points A, B and C on the circle. Join AB and BC. Draw the ⊥ bisectors of line segments AB and BC. The two ⊥ bisectors intersect at O. Then, O is the centre of the circle.     OA = OB and OB = OC, [ Points lying on the ⊥ bisector of the line segment joining the two points are equidistant from them.] i.e., OA = OB = OC is equal to the radius of the circle.