# How does the covariance between two variables affect the variance of their sum?

$v a r \left[{X}_{1} \setminus \pm {X}_{2}\right] = v a r \left[{X}_{1}\right] + v a r \left[{X}_{2}\right] \setminus \pm 2 c o v \left[{X}_{1} , {X}_{2}\right]$