radical, in mathematics, symbol () placed over a number or expression, called the radicand, to indicate a root of the radicand. When used without a sign or index number, as in , it designates the positive square root of the radicand, i.e., 2. If both square roots are meant, the radical sign is preceded by ±, as in ±. To indicate higher roots of the radicand, e.g., cube or fourth roots, an index number is used, as in . The radical sign is generally taken to indicate the principal root of the radicand, i.e., = 3, although any radicand will have n different nth roots. The term radical is sometimes used loosely to refer to the entire expression consisting of radical sign and radicand.
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