User:Gmac4247/basic-geometry/Perimeter of a circle

Perimeter (circumference) of a circle
The perimeter (P) of a circle is the difference between the circle and a smaller, concentric circle divided by the difference of the radius of those circles (x).

Note: Unwrapping that ring would result in a trapezoid shape, but with just a theoretic height, close to zero. Therefor, it can be regarded as a rectangle.

$$ P=[3.2r^2 - 3.2(r-x)^2]/x=6.4r-x $$ As x is almost zero, P=6.4r

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