How to make the limit (mathematics) sign?

You are looking for \lim_{x \to 2} f(x) = 5. This has to be used in math mode which can be either inline mode (where the limit is placed as a subscript so that the inter line spacing of the paragraph is not perturbed):

or in display mode where the limits are placed underneath):

References:

• For the two types of math modes, please see:

  1. Display Math: Why is \[ ... \] preferable to $$ ... $$?
  2. Inline Math: Are \( and \) preferable to dollar signs for math mode?

• An excellent reference for math mode is Herbert Voss' comprehensive review of mathematics in (La)TeX

Code:

\documentclass{article}

\begin{document}
With in line mode this is typeset as $\lim_{x \to 2} f(x) = 5$

\medskip
In display mode it is typset differently:
\[ \lim_{x \to 2} f(x) = 5 \]
\end{document}

You can also use:

$\displaystyle{\lim_{x \to \infty}}$

It's in the inline mode, plus the limit is placed underneath.