The LaTeX command for triangle (△) is \triangle (triangle / Laplacian).
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Write \triangle 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.
\triangle is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing △ in a formula gives the same symbol.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\Re\Im\alephx'x''\angle\measuredangle\diamond\bigodot\bigoplus\bigotimes\mathcal{L}\mathcal{H}\mathcal{F}More math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \triangle in math mode; it needs no extra package. For example, $\triangle$.
None: \triangle is built into LaTeX, so no extra package is required.