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4. If a line intersects two concentric circles (circles with the same centre) with centre O at A, B, C and D. prove that AB = CD (see figure).
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Answer :

Let OP be the perpendicular from O on linel . Since, the perpendicular from the centre of a circle to a chord bisects the chords.
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Now, BC is the chord of the smaller circle and \(OP\perp{BC}\).
Therefore, BC = PC ...(i)
Since, AD is a chord of the larger circle and \(OP\perp{AD}\).
Therefore, AP = PD ...(ii)
On subtracting Eq. (i) from Eq.(ii), we get,
AP - BP = PD - PC
Thus, AB = CD
Hence, it is proved.