Determine whether the following statements are true and give an explanation or counterexample.

a. Consider the linear function f(x) = 2x + 5 and the region bounded by its graph and the x-axis on the interval [3, 6]. Suppose the area of this region is approximated using midpoint Riemann sums. Then the approximations give the exact area of the region for any number of subintervals.

b. A left Riemann sum always overestimates the area of a region bounded by a positive increasing function and the x-axis on an interval [a, b].

c. For an increasing or decreasing non constant function on an interval [a, b] and a given value of n, the value of the midpoint Riemann sum always lies between the values of the left and right Riemann sums.