The LaTeX command for arccos is \arccos.
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Write \arccos 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.
\arccos sets the name upright with operator spacing; typed as plain letters it would print in italics, as a product of variables.
$\arccos_{i=1}^{n} x_i$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\operatorname*{argmax}\operatorname*{argmin}\operatorname{lcm}\sin\cos\tan\arcsin\arctan\sinh\cosh\tanh\cot\sec\cscMath operators in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \arccos in math mode; it needs no extra package. For example, $\arccos_{i=1}^{n} x_i$.
None: \arccos is built into LaTeX, so no extra package is required.