Isoperimetric inequality

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Statement

Let \gamma be a simple closed curve in the Euclidean plane \R^2. Let Int(\gamma) denote the interior of \gamma. Let l(\gamma) denote the length of \gamma. Then:

4\pi Ar(Int(\gamma)) \le l(\gamma)^2

where Ar is the area.

Equality holds if and only if \gamma is a circle.