The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r:
$${\displaystyle A=\pi R^{2}-\pi r^{2}=\pi \left(R^{2}-r^{2}\right).}$$
The area of an … See more

In complex analysis an annulus ann(a; r, R) in the complex plane is an open region defined as
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