Area of a Circle Formula
The formula follows from limits of inscribed polygons or from the disk integral in polar coordinates. For school work, memorize it and track units.
Variables
- r — Radius
When to use it
Any time you need the interior measure of a disk and you know the radius or diameter.
Worked example
r = 5
- 25π
Answer: 25π
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Example problems
Common mistakes
Using diameter in place of r
If d = 10, then r = 5, so A = 25π, not 100π.
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Frequently asked questions
Circumference formula?
C = 2πr, which is the boundary length, not the area.