The useful life (in hours) of a certain kind of integrated circuit is a random variable having the probability density
![](https://test.transtutors.com/qimg/165c024b-ec5b-4b4e-a297-cd0a24aa9ac8.png)
If three of these circuits operate independently, find
(a) the joint probability density of X1, X2, and X3, representing the lengths of their useful lives;
(b) the value of