Chapter 5 - Section 5.7 - Factoring by Special Products - Exercise Set - Page 310: 79

$A=\pi( R+r)( R-r)$

Using $A=\pi r^2$ or the area of a circle, the area of the outer circle is $A=\pi R^2$ and the area of the inner circle is $A=\pi r^2.$ Subtracting these two areas, then the area of the washer is \begin{array}{l}\require{cancel} A=\pi R^2-\pi r^2 \\ A=\pi( R^2-r^2) \\ A=\pi( R+r)( R-r) .\end{array}