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concentric circles, A on larger, midpoint of BC on smaller, ABC equilateral

Source: Sharygin 2005 VIII-IX CR 7

August 18, 2019
EquilateralEquilateral Triangleconcentric circlesgeometryangles

Problem Statement

Two circles with radii 11 and 22 have a common center at the point OO. The vertex AA of the regular triangle ABCABC lies on the larger circle, and the middpoint of the base CDCD lies on the smaller one. What can the angle BOCBOC be equal to?
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