4-64. For each of the following moment-generating functions, find the associated probability density function:

(a) mX(t) = e7t−e3t 4t , t
= 0

(b) mX(t) = (1 − 0.8t)−1, t ≤ 1.25

(c) mX(t) = 2 2−t , t < 2