Students form a conjecture about the ratio of two areas in a quadrilateral, and use Sketchpad to search for a proof. In the process they verify or disprove their results for two different kinds of quadrilaterals. The link to the tool itself is to a zip file that contains both the sketch and a pdf file of the corresponding activity.

The combined sketch and pdf activity file are also available in .sit format. This is a free sample activity from Rethinking Proof with the Geometer's Sketchpad.