In a quadrilateral ABCD, AO and BO are the bisectors of ∠A and ∠B respectively. Prove that ∠AOB=12(∠C+∠D).

In a quadrilateral ABCD, AO and BO are
the bisectors of $∠ A$ and $∠ B$ respectively. Prove that $∠ A O B = \frac{1}{2} (∠ C + ∠ D) .$

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