Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 368: 78

$x^4-25$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the product of the given expression, $ (x^2+5)(x^2-5) ,$ is \begin{array}{l}\require{cancel} (x^2)^2-(5)^2 \\\\= x^4-25 .\end{array}