Use [ tex ] ... [ /tex ] around your formulas to display them mathematically (without the spaces before and after "tex").
To get a summation, write [ tex ]\sum_{i=0}^{\infty} x_i[ /tex ], which shows up like:
To get a limit, write [ tex ]\lim_{n \to \infty} x_i[ /tex ], which shows up like:
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