The area of a trapezoid that has two parallel bases of lengths B1 and B2 separated by distance H is given by the formula:
area = (B1+B2)*H/2
For example, a trapezoid having parallel sides of lengths 6 and 8 and height equal to 4 would have an area of (6+8)*4/2 = 28. As another example, suppose the price of a good increases from $10 to $15 and the quantity demanded falls from 80 to 60, as shown in the diagram below. Consumer surplus would decrease by the shaded area, which is equal to (60+80)*5/2 = $350.