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O is the center of the circle, and the measure of ∠O is 70°.What is the measure of ∠OAB?

Explanation

Let x = m∠OAB. OA = OB since radii of the same circle have equal measures. Therefore, m∠OAB = m∠OBA.
x + x + 70 = 180
2x + 70 = 180
2x = 180 − 70 = 110
x = 110 ÷ 2 = 55

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