**Data:** A line intersects two concentric circles with centre O at A, B, C and D.

**To Prove:** AB = CD.

**Construction:** Draw OM⊥AD.

**Proof:** AD is the chord of bigger circle. OM⊥AD

∴ AM = MD ………(i)

(∵ Perpendicular bisects the chord from centre)

Similarly, BC is the chord of smaller circle. OM⊥BC.

∴ BM = MC ………. (ii)

Subtracting (ii) from (i),

AM – BM = MD – MC

∴ AB = CD.