59. Let X1 denote the time (hr) it takes to perform a rst task and X2 denote the time it takes to perform a second one. The second task always takes at least as long to perform as the rst task. The joint pdf of these variables is

a. Obtain the pdf of the total completion time for the two tasks.

b. Obtain the pdf of the difference X2  X1 between the longer completion time and the shorter time.

f 1x1, x2 2  e 21x1  x2 2 0 x1 x2 1

0 otherwise