\sqrt[root]{arg}
The \sqrt command produces the square root (radical) symbol with the argument as radicand. The optional argument, root, determines what root to produce, i.e. the cube root of x+y would be typed as $\sqrt[3]{x+y}$.
The radical sign is variable size, that is, it is made larger if the radicand is, for example, a built-up fraction.
See also: Math Formulas,
Math Miscellany.
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