<t>By linearity of covariance you would have cov(x1, x2)=ac cov(r1, r1)+ bd cov (r2, r2) + (ad+bc)cov(r1, r2) = ac (v1)+ bd (v2) + (ad+bc)cov(r1, r2).You need to know the covariance between r1 and r2, knowing only their variances is not enough to do the question. By the way, you also need to know th...