Back to Unit 9

Area of a Trapezoid


If b1 and b2 are the lengths of the two parallel bases of a trapezoid, and h is its height, the area of the trapezoid is
1/2  h 
  (b1 + b2) .    **** Remember the two bases are always parallel to each other and the

                                              height is perpendicular to both bases

To picture this, consider two identical trapezoids, and "turn" one around and "paste" it to the other along one side as pictured below:

The figure formed is a parallelogram having an area of h  (a + b), which is twice the area of one of the trapezoids.


What is the area of a trapezoid having bases 12 and 8 and a height of 5?
Using the formula for the area of a trapezoid, we see that the area is
  5   (12 + 8) = 1/2   5   20 = 1/2   100 = 50.