In Problems 13–22, a function f is nonnegative and continuous on an interval [a, b].
(a) Graph f, indicating the area A under f from a to b.
(b) Approximate the area A by partitioning [a, b] into four subintervals of equal width and choosing u as the left endpoint of each subinterval.
(c) Approximate the area A by partitioning [a, b] into eight subintervals of equal width and choosing u as the left endpoint of each subinterval.
(d) Express the area A as an integral.
(e) Use a graphing utility to approximate the integral.

f(x) = x, [0, 4]