[color=#000000][b]Theorem:[/b][/color][br][br]If a secant and a tangent are drawn to a circle from a point outside the circle, then the product of the secant segment and its external segment is equal to the square of the tangent segment. In essence, for the applet above, we can write [math]b\cdot ext_b=t^2[/math]. [br][br]This comes from the fact that triangle [i]ACB[/i] and triangle [i]DCA [/i]are similar by the AA~ Theorem. [br]Can you write the step that comes next in the proof? [br]