Refer to the illustration. The interval [1, 3] is partitioned into two subintervals and [1, 2] [2, 3]. 

Approximate the area A choosing u as the left endpoint of each subinterval.

3 f(x) f(3) = 4 f(2) = 2 f(1) = 1 1 2 3 f(1) = 1, f(2) = 2, f(3) = 4