Let Y1 and Y2 be uncorrelated random variables. Consider U1 = Y1 + Y2 and U2 = Y1-Y2.
a. Find the covariance between U1 and U2 in terms of the variance of Y1 and Y2
b. Find the coefficient of correlation between U1 and U2 in terms of the variances (or std deviations) of Y1 and Y2
c. Is it possible that U1 and U2 would be uncorrelated? When would that happen?