# Math Help - Linear Combinations of Variance

1. ## Linear Combinations of Variance

Is this true?

$Var(5X_1-3X_2) = 5^2Var(X_1)+(-3)^2Var(X_2)$

or is it

$Var(5X_1-3X_2) = 5^2Var(X_1)-(3)^2Var(X_2)$

2. im afraid its neither unless X1 and X2 are uncorrellated.

if they are uncorrelated, it would be the first one....i think

3. $Var(5X_1-3X_2) = 5^2Var(X_1)+(-3)^2Var(X_2)+2(5)(-3)Cov(X_1,X_2)$