AB is a diameter of a circle with centre O. P is on BA extended, and PT is tangent to the circle. Use the corollary to the Intersecting Secants Property to prove that PT is perpendicular to OT.

Here is the diagram (Without the dashes in the "com" and periods instead of the commas, in the following address.:
i72,photobucket,c-o-m/albums/i162/The_Showman/10,jpg

The intersecting secants property is "If two secants AB and CD intersect at point P, then PA dot PB = PC dot PD.

The corollary to that is "If a tangent PT is drawn from a point on a secant AB, then PA dot PB = PT^2."