Thales’s Theorem: A triangle formed by a circle’s diameter and any point on the semicircle is always a right-angled triangle (the angle opposite this diameter is a right angle (90°)).
Sum of interior angles in triangles: Remember, the sum of interior angles in any triangle is always $180^\circ$, allowing you to solve for unknown angles by subtraction.
Using Thales’s Theorem to solve problems: Knowing that $\angle C = 90^\circ$ simplifies calculations, especially in complex diagrams with multiple triangles.
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