Q. 73.7( 3 Votes )

Two circles with centers A and B touch each other internally. Another circle touches the larger circle externally at the point x and the smaller circle externally at the point y. If O be the centre of that circle, let us prove that AO + BO is constant.

Answer :


Let the radius of the circle with centre A be Ra, B be Rb and O be Ro

Length OA = Radius of circle O + Radius of circle A

OA = Ro + Ra

Length OB = Radius of circle O + Radius of circle B

OB = Ro + Rb

AO + BO = Ro + Ra + Ro + Rb

AO + BO = 2Ro + Ra + Rb

Since the radius is always a constant quantity so AO + BO is also a constant quantity.

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