A function f is positive, increasing, and concave down on the interval [1,5]. The interval is divided into four equal subintervals of length 1. Four different numerical estimates for ∫15f(x)dx are obtained:
I. L=23.4 (using left–endpoint rectangles)
II. R=31.2 (using right–endpoint rectangles)
III. M=27.8 (using midpoint rectangles)
IV. T=27.3 (using trapezoids)
Which estimate is most likely to be an overestimate of the true value?
a) III only
b) IV only
c) II only
d) II and III