Annulus
Find the area of the annulus (purple ring) based on the given the chord length of CD.
When you drag B to change the size of the outer circle, does the area of the purple ring increase or decrease?
Annulus
You may imagine a circular building, such as the Hopewell Centre in Hong Kong. The purple ring below could be the cross section of the building, showing the a circular corridor. Suppose you are going to find the area of this corridor during renovation, but cannot access the centre of this building. It is interesting to find that measuring a chord (like CD) is enough to calculate the area.