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Definition

Index of the Radical

In the radical notation an\sqrt[n]{a}, the number nn is called the index of the radical. The index specifies which root is being taken — it tells you the power to which some number must be raised to produce the radicand aa.

  • For a square root, the index is 22, but by convention it is not written: a\sqrt{a} means a2\sqrt[2]{a}.
  • For a cube root, the index is 33: a3\sqrt[3]{a}.
  • For a fourth root, the index is 44: a4\sqrt[4]{a}.
  • For a fifth root, the index is 55: a5\sqrt[5]{a}.

The index must be a positive integer greater than or equal to 22. Together with the radicand (the expression under the radical sign), the index completely determines the value of a radical expression.

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Updated 2026-04-21

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