How can you use the formula for area of a parallelogram to determine the area of a trapezoid if you forgot the formula for area of a trapezoid?

You can put two identical trapezoids together to form a parallelogram with the same height as the trapezoid and a base length equal to the sum of the base lengths of the trapezoid. The area of the parallelogram is (b1 + b2) h, so the area of the trapezoid is one-half of this area.