Drag the angle slider to rotate a copy of the triangle 180° around the midpoint of a side.[br]These two triangles make a parallelogram with the same base and height as the original triangle.[br]When the angle reaches 180°, a new slider will appear. Drag this to 1.[br]This cuts the parallelogram into two congruent pieces, then slides one of them along the base to make a rectangle with the same base and height as the original triangle.[br][br]Conclusion: The area of a triangle is half the area of a rectangle with the same base and height.[br][br]To use this fact to find the area of a triangle[br]choose a base for the triangle (this applet shows a particular base, but you can use any side)[br]Measure the length of the base and the height of the triangle, perpendicular to the base.[br]Multiply to get the area of the rectangle.[br]Take half of that to get the area of the triangle.