Is this true? $\displaystyle Var(5X_1-3X_2) = 5^2Var(X_1)+(-3)^2Var(X_2)$ or is it $\displaystyle Var(5X_1-3X_2) = 5^2Var(X_1)-(3)^2Var(X_2)$
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im afraid its neither unless X1 and X2 are uncorrellated. if they are uncorrelated, it would be the first one....i think
$\displaystyle Var(5X_1-3X_2) = 5^2Var(X_1)+(-3)^2Var(X_2)+2(5)(-3)Cov(X_1,X_2)$
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