Math
posted by rmz .
Garden Area Problem. A designer created a garden
from two concentric circles whose equations are as follows:
(x+2)^2+(y6)^2=16 and (x+2)^2+(y6)^2=81
The area between the circles will be covered with grass. What is the area of that section?
How do you do this?

The equation of a standard circle with centre at (a,b) and radius r is
(xa)² + (yb)² = r²
By inspection of the given circle,
(x+2)^2+(y6)^2=16 and (x+2)^2+(y6)^2=81
we conclude that both have centres at (2,6), therefore they are concentric.
The radii of the circles are √16=4 and √81=9.
The area between the two circles are therefore
πr1²πr2²
=π(9²4²)
=65π 
do you have any sites that explain this as well?

The subject should be in any precalc textbook. Alternatively, you can google
"equation of a circle".