The theorem states that the measure of an angle inscribed in a circle is half the measure of the arc it subtends. Let ∠ABC be an inscribed angle subtending arc AC in a circle with center O.
Step 1: Prove the theorem for the case where one side of the inscribed angle passes through the center O.
Assume side BC is a diameter. Draw radius OA.
In △OAB, OA=OB because they are both radii of the same circle, making △OAB an isosceles triangle.
Therefore, the base angles are equal: m(∠OAB)=m(∠OBA).
The central angle ∠AOC is an exterior angle to △OAB.