The applet visualizes the formula:
Area of Annulus = (Average Circumference)*(Thickness).
This can be particularly useful for illustrating the volume of thin cylindrical shells in Integral Calculus.
References: https://www.geogebra.org/m/AYUSaqEB and
https://www.quora.com/Can-one-calculate-the-volume-of-a-torus-by-pretending-its-a-cylinder/answer/Job-Bouwman .
