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?
The area of annular section is Pi*(R^2r^2)
and we know l^2/4 +r^2=R^2
annular area = Pi*(l^2/4) where l is the lenght of the tangent

Posted by salil
on 20060225 07:13:42 