Geometrie-Kreis-Kreisring

$A = (r_{a} ^{2} - r_{i} ^{2} )\cdot\pi$
1 2 3
$r_{a} = \sqrt{\frac{A}{\pi } + r_{i} ^{2} }$
1
$r_{i} = \sqrt{r_{a} ^{2} - \frac{A}{\pi } }$
1