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G
B
$ A = (r_{a} ^{2} - r_{i} ^{2} )\cdot\pi $
$ r_{a} = \sqrt{\frac{A}{\pi } + r_{i} ^{2} } $
$ r_{i} = \sqrt{r_{a} ^{2} - \frac{A}{\pi } } $
Geometrie-Kreis-Kreisring