Posted by rmz on Friday, February 19, 2010 at 5:39pm.
The equation of a standard circle with centre at (a,b) and radius r is
(x-a)² + (y-b)² = r²
By inspection of the given circle,
(x+2)^2+(y-6)^2=16 and (x+2)^2+(y-6)^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
πr1²-πr2²
=π(9²-4²)
=65π
do you have any sites that explain this as well?
The subject should be in any pre-calc textbook. Alternatively, you can google
"equation of a circle".
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