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A radical is the mathematical symbol $\sqrt{\;\;}$ (modified Latin $r$). It denotes the extraction of a root, that is, the solution of a two-term algebraic equation of the form $x^n-a=0$. The symbol $\sqrt[n]a$ denotes one of the roots of this equation.
The problem of the solution of algebraic equations over the complex field $\mathbf C$ in radicals is to express the roots of an algebraic equation (with complex coefficients) in terms of its coefficients using a finite number of additions, multiplications, divisions, raising to a power, and root extractions. Equations of degree higher than 4 cannot, in general, be solved by radicals (see Galois theory).