Develop an expression for the reliability of the standby system shown in the following figure. Components $1$ and $2$ are operated in a series manner, same as components $3$ and $4.$ When the component $1$ or $2$ fail, the changeover switch brings components $3$ and $4.$ is the failure rate of components $1$ and $2$ and ${}_s$ is the failure rate of components $3$ and $4.$ There is no failure when component is in standby $($cold standby$).$

$2.$ What's the system's MTTF$?$

$3.$ Discuss if components $3$ and $4$ are on hot standby $($parallel structure$),$ how the system's reliability function may change.