Solution :- Construction: Draw two circles having centres O and O' intersecting at points A and B. Draw a parallel line PQ to OO' Join OO',OP,O'Q,OM and O'N To Prove: PQ = 2OO' Proof: In △OPB, BM = MP ....(i) (Perpendicular from the centre to the circle bisect the chord) Similarly in △O'BQ, BN = NQ ....(ii) (Perpendicular from the centre to the circle bisects the chord) Adding (i) and (ii), BM + BN = PM + NQ Adding BM + BN to both the sides BM + BN + BM + BN = BM + PM + NQ + BN 2BM + 2BN = PQ 2(BM + BN) = PQ ....(iii) Again, OO' = MN [As OO' NM is a rectangle] ...(iv) ⇒ 2OO' = PQ Hence proved.