Let
h(x)=ax3−6x+4
for real a. Consider the interval [1,4].
a. Compute the average rate of change of h on [1,4] and write your answer as a function of a. Show all algebraic steps.
b. Find the instantaneous rate of change of h at x=1 in terms of a. Clearly indicate any derivative you compute.
c. Determine all real values of a for which the instantaneous rate of change found in part (b) equals the average rate of change found in part (a). Justify your solution algebraically.
d. For each value of a obtained in part (c), discuss whether there must exist at least one x in the open interval (1,4) at which the instantaneous rate of change of h equals that same common rate. Provide a clear justification based on properties of the function on the interval.