Consider the picture to the right. We have two concentric circles, one inside another. We do not know the radius for either one of them.

We do, however know that the chord (shown in red) - a line whose ends are on the outer circle and which is tangent to the inner circle - has a length of 10 inches.

What is the area (shaded in light-blue) between the two circles?

Imagine the radius of the inner circle is 0. Then the chord would equal the diameter of the bigger circle, hence making the area equal to pi * d ^ 2 / 4 or 25 * pi.

Posted by Tom on 2002-05-09 16:43:23