Two different computer systems are monitored for a total of n weeks. let xi denote the number of breakdowns of the first system during the ith week, and suppose the xi’s are independent and drawn from a poisson distribution with parameter l1. similarly, let yi denote the number of breakdowns of the second system during the ith week, and assume independence with each yi poisson with parameter l2. derive the mle of l1, l2, and l1 – l2. [hint: using independence, write the joint pmf (likelihood) of the xi’s and yi ’s together.]