index: click on a letter A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A to Z index index: subject areas numbers & symbols sets, logic, proofs geometry algebra trigonometry advanced algebra & pre-calculus calculus advanced topics probability & statistics real world applications multimedia entries
 Annulus Washer The region between two concentric circles which have different radii.   Area of annulus $$= \pi \left( {{R^2} - {r^2}} \right)$$