Thales' Theorem/Proof 2

Proof
Let $D$ be the center of $ACB$.

From the Inscribed Angle Theorem, $\angle ADB = 2 \angle ACB$.

Then we have that $\angle ADB$ is a straight angle.

Hence the result.

Legend has it that he sacrificed an ox in honour of the discovery.

On the other hand, some attribute this theorem to.