For the $\displaystyle M/M/1 $ queue, show that the probability that a customer spends an amount of time $\displaystyle x $ or less in a queue is given by: $\displaystyle \begin{cases} 1- \frac{\lambda}{\mu}, \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{if} \ x=0 \\ 1- \frac{\lambda}{\mu}+\frac{\lambda}{\mu}(1-e^{-(\mu-\lambda)x}), \ \text{if} \ x > 0 \end{cases} $