f(x) = squaroot(x) -(1/7)x
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Learn LaTeX. While you're in the editing window, at the end of the 2nd row of formatting/functions (the one that starts with bold, italics, underline), you'll see the TeX button. Click it, and the math tags appear:
[ MATH ][ /MATH ]
(I added extra spaces so that the tags would appear.) You'll have to learn the syntax to type the equations in. Look at the LaTeX Help subforum. Here's what I typed so that the original equation would appear:
[ MATH ]f(x) = \sqrt{x} - \frac{1}{7}x[ /MATH ]
(again, without the extra spaces).