Remember this problem? The one where you had to find the area between two circles by only knowing the length of the tangent "chord" (the red line)?

That problem is solved using some (moderately) tricky geometry. But if you know that the problem is solvable, it's actually pretty easy to solve by simply using the formula for the area of the circle and a bit of logic.

Can you do it?

This time, you are (basically)told that the area is a constant, rather than a function of the two radii, so the elegant soltion is valid:

Assume a radius of 0 for the smaller circle. The "ring" and the larger circle become identical, the chord becomes a diameter and the area becomes pi(R^2) = pi(5^2) = pi(25) = 25pi

Posted by TomM on 2002-05-10 07:24:18