P and Q are points on intersection of two circles with centre O and O'. If straight line APB and CQD are parallel to OO' then AB =
Join PQ,
Draw OM' ⊥ CQ, OM ⊥ AP, O'N' ⊥ QD and O'N ⊥ PB.
MP = 12 AP, NP = 12 BP, M'Q = 12 CQ, and N'Q = 12 QD.
[∵ A perpendicular drawn from the centre of a circle to its chord bisects the chord]
⇒ OO' = MN = MP + PN= 12( AP + BP) = 12 AB.......(i)
Similarly,
OO' = M'N' = M'Q + QN' = 12( CQ + QD) = 12 CD.......(ii)
From (i) and (ii), we can see that AB = CD.