Given: O is the circumcentre inside the ΔDEF. ∠EOF = 150° Concepts used: The measure of the angle formed with the given base at circumcentre = 2 × vertex angle formed opposite to the same base Calculation: ⇒ ∠EOF = 2 × ∠D ⇒ 150° = 2 × ∠D ⇒ ∠D = 150°/2 ⇒ ∠D = 75°. ∴ The measure of vertex ∠D is 75°.