1. The problem is to state the Circle Theorem. 2. The Circle Theorem states that the angle subtended by a diameter of a circle at any point on the circle is a right angle (90 degrees). 3. This means if you have a triangle inscribed in a circle where one side is the diameter, the angle opposite that side is always 90 degrees. 4. This theorem is useful in geometry to prove right angles and properties of circles. 5. In formula form, if $\angle ABC$ is subtended by diameter $AC$ of the circle, then: $$\angle ABC = 90^\circ$$ 6. Remember, this only applies when the side is the diameter of the circle. 7. This is a fundamental property of circles and is often used in problems involving inscribed angles and right triangles.