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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. 

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θ=60°=π/3radians
Arc length =θr where θ is in radians
π/3 × r + π/3×(r+7) +7+7 =28 2/3
/3r =14 2/3/3
        r=3.502

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