Let $Y_1$ and $Y_2$ be uncorrelated random variables. Let the means and variances of $Y_1$ and $Y_2$ are ${}_1,2{}^2$ and ${}_2,{}^2$ respectively. Consider $U_1=Y_1+Y_2$ and $U_2=Y_1-Y_2.$ The Cov$(U_1,U_2)$ in terms of the variances of $Y_1$ and $Y_2$ is: