Chapter R - Test - Page 78: 27

$x-y$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like term and using the properties of radicals, the product of the given expression, $ (\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y}) ,$ is \begin{array}{l}\require{cancel} (\sqrt{x})^2-(\sqrt{y})^2 \\\\= x-y .\end{array}