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angle wanted, equilateral, AN = BM, 3MB = AB

Source: 2010 Thailand Mathematical Olympiad day 1 p4

March 12, 2021
geometryEquilateralequal segments

Problem Statement

Let △ABC\vartriangle ABC be an equilateral triangle, and let MM and NN be points on ABAB and ACAC, respectively, so that AN=BMAN = BM and 3MB=AB3MB = AB. Lines CMCM and BNBN intersect at OO. Find ∠AOB\angle AOB.
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