\begin{block}{Formulas for the variance of random variables} If $X$ and $Y$ are two independent random variables then \begin{align*} Var[X+Y] = Var[X] + Var[Y] \\ Var[X-Y] = Var[X] + Var[Y] \end{align*} \end{block}
Examples
What is the variance of the result of tossing two dies?
We use $X$ and $Y$ to represent tossing a single die. The variance of $X$ and $Y$ is 2.92 (calculated in the lecture on random variables).