Adevice contains two circuits. the second circuit is a backup for the first, so the second is used only when the first has failed. the device fails when and only when the second circuit fails. let x and y be the times (in years) at which the first and second circuits fail, respectively. their joint probability density function (pdf) is 6e-xe-20 for 0 < x < y 〈 oo; otherwise (a) find the pdf of x. identify the distribution of x (b) find the pdf of y (c) what is the expected time at which the device fails?