Let f be a positive, decreasing function on [1,5]. A sum is formed by multiplying the width of each subinterval in a partition 1=x0<x1<x2<x3=5 by the value of f at the left endpoint of that subinterval and then adding the results. Which statement best describes this sum relative to ∫15f(x)dx?
a) It is an underestimate because f is decreasing on [1,5].
b) It could be an overestimate or an underestimate depending on the concavity of f on [1,5].
c) It is an overestimate because f is decreasing on [1,5].
d) Whether it is an overestimate or an underestimate cannot be determined without a uniform partition.