In a circle with center O, diameters AD and BC are drawn, angle OCD is 30 °. Find the value of the angle OAB.

univerkov | April 29, 2021 | education

Consider triangles AOB and COD, they are isosceles, the lateral sides are the radii of the given circle, the angles at the base are equal. The apex angles AOB and COD are vertical. We get the equality of triangles according to the first criterion (two sides and an angle).
∠OAB = ∠OBA = ∠OCD = ∠ODC = 30 °.
Answer: the value of the angle OAB is 30 °.

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