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To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\Var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

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

To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\Var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

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

To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\Var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

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

To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\Var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

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

To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}}\Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}}\Var[x]+\Var[y]$$

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

To typeset an operator, it is much better to use \operatorname instead of \mathrm, as explained on tex.SE: What's the difference between \mathrm and \operatorname?

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

As noted by @cardinal, MathJax supports \newcommand. 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 \newcommand{\Var}{\operatorname{Var}}, e.g.

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

and this will be displayed as

$$\newcommand{\Var}{\operatorname{Var}} \Var[x]+\Var[y]$$

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