Let x1 and x2 denote the proportions of time, out of one workweek, that employees i and ii, respectively, actually spend performing their assigned tasks. the joint relative frequency behavior of x1 and x2 is modeled by the pdf f(x1, x2) = ( x1 + x2 for 0 ≤ x1 ≤ 1, 0 ≤ x2 ≤ 1 ), 0 otherwisea). find cov(x1, x2).b). find e(3x1 − 2x2).c). find v (3x1 − 2x2).