Chapter 5 - Section 5.3 - Special Products - Exercise Set - Page 367: 37

$y^4-1$

Using $(a+b)(a-b)=a^2-b^2$ or the special product of the sum and difference of like terms, the product of the given expression, $ (y^2+1)(y^2-1) ,$ is \begin{array}{l}\require{cancel} y^2(y^2)-1(1) \\\\= y^4-1 .\end{array}