Proof for the Area of a Circle - Geometry Made Easy
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Proof that A=πr2

circle divided into sections like an orange

We know it's true that A=πr-squared is the area of a circle, but how can we prove that it is always true for every imaginable circle? Here's how Euclid did it.

Start by dividing a circle into sixteen sections like an orange. We know that together they add up to the area of the whole circle.

Now take all the green sections and line them up next to each other like the bottom teeth of some wild animal. On top of them, line up all the orange pieces like the top teeth of the animal, and then fit them together as if the animal had closed his mouth.

Area

All the teeth together look almost like a rectangle. The short side of this rectangle is the radius of the circle. The long side of the rectangle is half of the circumference of the circle, or 2πr. If we multiply them together to get the area of the rectangle, we get r (πr) or πr-squared - the area of a circle.

If we cut our circle into smaller sections, our rectangle will be straighter, but this is enough to see what it would be like.

More about Geometry

Bibliography and further reading about circles:

More Geometry
More about Math
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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