# Math Help - computing variance

1. ## computing variance

in the last line of the above, why is the last expression not 2ab sigma xy? why is it just ab when the line above says 2abCov(X,Y)

2. ## Re: computing variance

$\sigma_{XY} = Cov(X,Y)$ Its just notation. For example $\sigma_x = Cov(x,x) = \sigma_{XX}$
Sorry i miss read you question. It looks like they have a typo.