Chapter 7 - Section 7.3 - Adding and Subtracting Rational Expressions with the Same Denominator - Concept and Vocabulary Check - Page 506: 3

$\displaystyle \frac{-1}{-1}$

When one denominator is the opposite, or additive inverse, of the other, first multiply either rational expression by $\displaystyle \frac{-1}{-1}$ to obtain a common denominator. (We are multiplying with 1, so the value of the chosen expression does not change)