Chapter 8 - Section 8.2 - Multiplying and Dividing Radicals - Exercise Set - Page 585: 5

$5$

Using $\sqrt{x}\cdot\sqrt{y}=\sqrt{xy}$ or the product rule of radicals, the given expression, $ \sqrt{5}\cdot\sqrt{5} ,$ is equivalent to \begin{array}{l}\require{cancel} \sqrt{5(5)} \\\\= \sqrt{(5)^2} \\\\= 5 .\end{array}