# Chapter 5 - Section 5.7 - Negative Exponents and Scientific Notation - Exercise Set - Page 408: 20

$25$

#### Work Step by Step

Using $x^{-a}=\dfrac{1}{x^a}$ the given expression, $\left( \dfrac{1}{5} \right)^{-2} ,$ simplifies to \begin{array}{l}\require{cancel} \dfrac{1}{\left( \dfrac{1}{5} \right)^{2}} \\\\= \dfrac{1}{\left( \dfrac{1}{25} \right)} \\\\= 1\div\dfrac{1}{25} \\\\= 1\cdot\dfrac{25}{1} \\\\= 25 .\end{array}

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