Hands-on math!


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 (R2) minus the area of the inner circle (r2).
So, if R = 6 inches and r = 4 inches, then the blue area (called an 'annulus' or 'annular ring') is
    62 - 42 =
    (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.