In the figure below, AB and CD are arcs of two concentric circles, centre O. Angle AOB = 60° and AD=BC=7cm.Given that the perimeter of ABCD is 28 ^{2}/_{3} cm, find OA, the radius of the inner circle.

θ=60°=π/3radiansArc length =θr where θ is in radians^{π}/_{3} × r + ^{π}/_{3}×(r+7) +7+7 =28 ^{2}/_{3}^{2π}/_{3r} =14 ^{2}/_{3}− ^{7π}/_{3} r=3.502

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