
The area of a closed curve at a fixed offset from another curve - alejohausner
https://math.stackexchange.com/questions/1402201/prove-the-theorem-on-analytic-geometry-in-the-picture
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jonsen
Tangentially:

[https://en.m.wikipedia.org/wiki/Visual_calculus](https://en.m.wikipedia.org/wiki/Visual_calculus)

