Any triangle can be rotated about the midpoint of one of its sides to form a parallelogram. If a height and base is given, rotate the triangle about one of the sides that is not the base. The height of the parallelogram will then correspond to the altitude of the triangle.

The formula for the area of the interior of a parallelogram is A = bh, where "b" is the length of the base of the parallelogram and "h" is the length of the height of the parallelogram. Since the height of the parallelogram is equivilant to the altitude of the triangle that generated it, the formula for the area of the parallelogram could be thought of as A = ab.

Since the triangle is half the area of the parallelogram, the formula for the area is: