Area of an Annulus

Definition of Annulus of a Circle

An [b]annulus of a circle[/b] is the region between two concentric circles. Recall, concentric circles are circles that have the same center.[br][br]Move the "outer" point until the large radius is 5 units.[br]Move the "inner" point until the small radius is 3 units.

In terms of pi, what is the area of the "outer circle" (r = 5 units)?

[math]5\pi[/math] square units [math]10\pi[/math] square units [math]25\pi[/math] square units

In terms of pi, what is the area of the "inner circle" (r = 3 units)?

[math]9\pi[/math] square units [math]3\pi[/math] square units [math]6\pi[/math] square units

Since we can find the area of the "outer circle" and "inner circle", we can easily find the area of the annulus too.

The [b]area of the annulus[/b] is simply the area of the ___________ circle subtracted by the area of the ____________ circle.

outer, inner inner, outer

Within our example, the area of the annulus is _______ square units.

[math]16\pi[/math] [math]2\pi[/math]