The LaTeX command for frac (⁄) is \frac{a}{b} (fraction).
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Write \frac{a}{b} inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
In display math \frac{a}{b} takes its full size; inline it shrinks to fit the line, and \displaystyle forces the large form. With unicode-math under XeLaTeX or LuaLaTeX, typing ⁄ in a formula gives the same symbol.
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Use \frac{a}{b} in math mode; it needs no extra package. For example, $\frac{a}{b} f(x)$.
None: \frac{a}{b} is built into LaTeX, so no extra package is required.