Circle proof 10: The tangent-secant theorem.

Prove the "tangent-secant theorem": When a secant and a tangent are drawn to a circle from an external point, the square of the length of the tangent equals the product of the lengths to the circle on the secant.

This is a direct consequence of the theorem about intersecting chords, when the intersection in outside the circle. Be careful to measure the lengths on the secant from the point of intersection, NOT the length of the part inside the circle (i.e. the chord).

Recorded during class when they were mowing outside.. sorry about the noise.