## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$-5$
Using $(a+b)(a-b)=a^2-b^2$, or the product of the sum and difference of like terms, and the properties of radicals, the given expression, $(\sqrt{10}-\sqrt{15})(\sqrt{10}+\sqrt{15}) ,$ is equivalent to \begin{array}{l}\require{cancel} (\sqrt{10})^2-(\sqrt{15})^2 \\\\= 10-15 \\\\= -5 .\end{array}