Let O be the centre of the two concentric circles and OM the perpendicular from O to the line l. AD is the chord of the larger circle and BC, the chord of the smaller circle.

Since, perpendicular from the centre to a chord bisects the chord, therefore,

AM = MD ...(i)

and BM = MC ...(ii)

(i) and (ii) gives, AM – BM = MD – MC

⇒** AB = CD**.