8. As shown in Figure 1, three small balls with a radius of $10 \mathrm{~cm}$ each are placed in a hemispherical bowl, with the tops of the balls exactly at the same level as the rim of the bowl. The radius of the bowl is $\qquad$ $\mathrm{cm}$.
$8.10\left(1+\frac{\sqrt{21}}{3}\right)$