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NL-NWO 2020 Problem 3

    Given is a parallelogram ABCDABCD with ∠A<90o\angle A < 90^o and ∣AB∣<∣BC∣|AB| < |BC| . The angular bisector of angle AA intersects side BCBC in MM and intersects the extension of DCDC in NN . Point OO is the centre of the circle through M,CM, C , and NN . Prove that ∠OBC=∠ODC\angle OBC = \angle ODC .

    ∘\circ ∘\circ AA BB CC DD MM NN OO

    Dataset text AI-MO AoPS. Quoted with source.

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