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

$a^2b^2-c^2d^4$
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $(ab+cd^2)(ab-cd^2) ,$ simplifies to \begin{array}{l}\require{cancel} (ab)^2-(cd^2)^2 \\\\= a^2b^2-c^2d^4 .\end{array}