How to calculate the area of a circle, plus simple proof for the formula
The formula for the area of a circle is A = πr2, or pi times r squared (radius squared). I show how to use the formula in several example situations. We then solve a few word problems. Lastly, I show a simple proof for the formula, based on dividing the circle into many sectors, which are close to triangles. Rearranging those sectors, we get a shape that is close to a parallelogram. Starting from the area of the parallelogram, we get the familiar formula for the area of a circle.
See also
Pi and the circumference of a circle — video lesson
Math Mammoth Geometry 2 — a self-teaching worktext with explanations & exercises (grades 4-5)
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