Concentric
Concentric means having the same center.
Two circles which have the same center are
concentric:
A common problem you see with concentric circles is like this:
What is the area of the blue ring in this diagram?
This is simply the area of the outer circle (R^{2}) minus the area of the inner circle (r^{2}).
So, if R = 6 inches and r = 4 inches, then the blue area (called an 'annulus' or 'annular ring') is
6^{2}  4^{2} =
(36  16) =
3.14 x 20 = 62.8 square inches.
Now, here's a problem for you:
What is the area of the ring in this figure? (measurements are in inches)
Area = _____  _____ = ______ square inches.

