Calculation formula of trapezoidal area. The reference answer on page 103 of the fifth grade math book is as follows. This is arrangement and review. It is a formula for calculating the area of parallelogram, triangle and trapezoid. As long as the formula is memorized, it is very simple to do the application problem. The area of a parallelogram is the base times the height. The area of a triangle is the base multiplied by the height and divided by 2. The area of a trapezoid is the sum of the upper and lower bottoms multiplied by the height and divided by 2.