1 of the Maths AA glossary’s terms, each with the part of the syllabus where you first meet it.
Finding the volume of a solid with a hole by slicing it into rings (washers) of outer radius RRR and inner radius rrr: V=π∫ab(R2−r2) dxV = \pi\int_a^b (R^2 - r^2)\,dxV=π∫ab(R2−r2)dx