tangent to the inner circle, in the accompanying diagram. That can be shown using the Pythagorean theorem since this line is tangent to the smaller circle and perpendicular to its radius at that point, so and are sides of a right-angled triangle with hypotenuse , and the area of the annulus is given by
:

The area can also be obtained via calculus by dividing the annulus up into an infinite number of annuli of infinitesimal