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?

25 pi

The area of the circle whose diameter is tangent to the inner circle and has endpoints at the outer circle is equal to the area of the annulus. The annulus being the area between the two concentric circles.

So if d = 10, r = 5

A = pi * r^2 = pi * 5^2 = 25 pi

Posted by Fizzle on 2002-05-08 12:00:27