Chapter 4 - Polynomials - 4.6 Special Products - 4.6 Exercise Set - Page 276: 24

$25-16x^{10}$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $ (5-4x^5)(5+4x^5) $, is equivalent to \begin{array}{l}\require{cancel} (5)^2-(4x^5)^2 \\\\= 25-16x^{10} .\end{array}