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index (of a radical)

The small number in the crook of a radical sign telling you which root to take.

The index of a radical $\sqrt[n]{a}$ is the number $n$ , specifying which root is meant — $n=2$ for a square root (usually left unwritten), $n=3$ for a cube root, and so on.

Examples

In $\sqrt[3]{8}$ , the index is $3$ .