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[SOLVED] simple geometry proof.

Let points A, B and C be on a circle O such that segment AB is a diameter of O (meaning the center of the circle is the midpoint of the segment). Prove m∠C = (1/2)Internal angle sum(ΔABC). Now this is not in Euclidean geometry . I don't know how to prove it and plus im limited since I haven't proven that the sum of the angles in a triangle add up to 180 and the exterior angle inequality where the angle outside a triangle adds up to the two interior angles. Instead all I have is the Sacceri Legendre theorem(all angles of a triangle add up less than 180*) and exterior angles( angle on the outside is greater than either interior angles). The rest of it is wide open.

Re: simple geometry proof.