Here is a simple sum on finding the area between two concentric circles.
http://in.youtube.com/watch?v=ihLK2_iaZ4w
The point to note in this example is that the first step in finding the area of any shape is to express the area as a sum or difference of regular shapes like circles, squares, rectangles etc. The same logic can be extended in 3-d to find the volumes of unusual shapes as well.
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