Caution! 1 Comment / Math Books, Math Pics / By mathfail 2.00 (1 votes)Loading... This is in a textbook: CAUTION!The area of a washer is pi*R^2-pi*r^2, which you can simplify to pi*(R^2-r^2), but not to pi*(R-r)^2. No matter how tempting it is to make the latter simplification, it’s wrong. Don’t do it.