Given: Two circles, with centres O and O' intersect at two points A and B. A AB is the common chord of the two circles and OO' is the line segment joining the centres of the two circles. Let OO' intersect AB at P. To prove: OO' is the perpendicular bisector of AB. Construction: Join OA, OB, O'A and O'B Proof: In triangles OAO' and OBO', we have OO' = OO' (Common) OA = OB (Radii of the same circle) O'A = O'B (Radii of the same circle) ⇒ △ OAO' ≅ △ OBO' (SSS congruence criterion) ⇒ ∠AOO' = ∠BOO' (CPCT) i.e., ∠AOP = ∠BOP In triangles AOP and BOP, we have OP = OP (Common) ∠AOP = ∠BOP (Proved above) OA = OB (Radio of the same circle) ∴ △AOR ≅ △BOP (By SAS congruence criterion) ⇒ AP = CP (CPCT) and ∠APO = ∠BPO (CPCT) But ∠APO + ∠BPO = 180° (Linear pair) ∴ ∠APO + ∠APO = 18 0° ⇒ 2∠APO = 180° ⇒ ∠APO = 9 0° Thus, AP = BP and ∠APO = ∠BPO = 9 0° Hence, OO' is the perpendicular bisector of AB.