Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 376: 69

$x^2y^2-a^2b^2$

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, $ (xy+ab)(xy-ab) ,$ is \begin{array}{l}\require{cancel} (xy)^2-(ab)^2 \\\\= x^2y^2-a^2b^2 .\end{array}