This zipped file contains a Geometer's Sketchpad file that can be used to explore the area of a triangle. The triangle can be altered by sliding the point B left and right. A related rectangle can be revealed. The triangle will have an area that is 1/2 of the area of the rectangle because the height and base of the triangle are the same as the height and base of the rectangle.
Various hide/show buttons allow you to reveal various information as you need it for the lesson.