Adjust the sliders shown near the lower right-hand corner to explore areas based on lower and upper bounds.
The "b" slider governs the distance between Lower and Upper bounds.
The rectangle at the bottom is a representation of the annulus if it were cut and stretched out.
This shape approaches that of a rectangle as the distance between L and U approaches zero.
For this reason, it is acceptable to estimate the area of each annulus as the product of a rectangle.
These individual areas can then be summed to estimate the area of the circle from which they originate.