THE AREA OF AN ANULUS

I recently came across this formula in the web and would like to share it with all of you.
An anulus is like a disc with a round hole in it. Although most people will think that you need loads of measurements but in actual fact you can get the area with just one measurement.

All you do is draw a straight line across the ring that just touches the little circle in the middle. If the length of this line is L then:
The area of the anulus is pi x (L/2)x (L/2)
But why?
The answer is rather satisfying.
Obviously the area is the same as the little circle taken away from the big circle, so let's see what happens when we try it.
Remember the area of a circle = PI x r2 where r is the radius of the circle (a radius is the distance from the middle to the edge).
Here we've called the radius of the big circle R and the little one is r.
The area of the big circle is PI x R2
The area of the little circle is PI x r2
So the area of the anulus is PI x R2 - PI x r 2 = PI x (R2 - r2)
Now we've got a right angled triangle.
Using pythagoras theorem, and you'll see that R2 = (L/2)x (L/2) + r2.
Therefore R2 - r2 = (L/2)x (L/2)
Swap this into our answer above and we get area of anulus= PI x (L/2)x (L/2)