In more detail
Circumference is 2πr and area is πr²: a circle of radius 5 has circumference 31.42 and area 78.54. Pi is irrational, so its decimals never end or repeat.
Glossary
Math & Statistics
Pi (π)
Pi is the ratio of a circle's circumference to its diameter, about 3.14159, the same for every circle.
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See also
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Glossary
Pythagorean theorem
In a right triangle the square of the hypotenuse equals the sum of the squares of the other two sides: `a² + b² = c²`.