Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 284: 57

$0.01-p^2q^2$

Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $ (pq+0.1)(-pq+0.1) ,$ simplifies to \begin{array}{l}\require{cancel} (0.1+pq)(0.1-pq) \\\\= (0.1)^2-(pq)^2 \\\\= 0.01-p^2q^2 .\end{array}