Chapter 10 - Exponents and Radicals - 10.2 Rational Numbers as Exponents - 10.2 Exercise Set - Page 641: 112

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Since $n$ is in the denominator of the exponent, it is a radical. Thus, if $n$ is even, the domain will only exist for numbers greater than or equal to zero, for one can only take the even root of positive or zero values. However, with odd roots, the domain will be all real numbers.