<p>To typeset an operator, it is much better to use <code>\operatorname</code> instead of <code>\mathrm</code>, as explained on tex.SE: <a href="https://tex.stackexchange.com/questions/48459/whats-the-difference-between-mathrm-and-operatorname">What's the difference between \mathrm and \operatorname?</a></p>

<p>Compare (first line uses <code>\mathrm</code>, second line uses <code>\operatorname</code>):
$$\mathrm{Var}[x] + 2\mathrm{Var}[y] - \mathrm{E}[x]\mathrm{E}[y]$$
$$\operatorname{Var}[x] + 2\operatorname{Var}[y] - \operatorname{E}[x]\operatorname{E}[y]$$
and notice small white spaces <em>before</em> the operator name in the second line.</p>

<p>As noted by @cardinal, MathJax supports <code>\newcommand</code>. The best way to use it is to put into the formula where the operator first appears, because putting it into a separate formula creates an annoying empty line (see @Zen's answer). So whenever you first need variance, you can type <code>\newcommand{\Var}{\operatorname{Var}}</code>, e.g.</p>

<pre><code>$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$
</code></pre>

<p>and this will be displayed as</p>

<p>$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$</p>

<p>All subsequent formulas can simply use <code>\Var</code> in them, e.g. <code>$\Var[z]$</code> will become $\Var[z]$.</p>
