Suppose that the number of typographical errors in a new text is poisson distributed with mean λ. two proofreaders independently read the text. suppose that each error is independently found by proofreader i with probability pi, i = 1, 2. let x1 denote the number of errors that are found by proofreader 1 but not by proofreader 2. let x2 denote the number of errors found by proofreader 2 but not by proofreader 1. let x3 be the number of errors found by both proofreaders. finally, let x4 denote the number of errors found by neither proofreader.(a) what is the expected value of xi (i=1, 2, 3, ) what is the expected value of xixj when i≠j (i, j=1, 2, 3, 4)?