It might seem silly now, but the area of a triangle can also be found with formula above. After all, a triangle is a trapezoid with only one base: b2 = 0. Similarly, a rectangle is a trapezoid with two equal bases: b1 = b2.
You can visualize the answer to this question. Do you see empty area between the parabola and the trapezoids, or do the trapezoids fill up more area than the parabola fills?