>>454
cC/Cb=cA/pAを
半径をxとして立式すると
https://www.wolframalpha.com/input?i=%282*x%2B9%2F7%29%2Fsqrt%28%28x%2B1%2F3%29%5E2-x%5E2%29%3Dsqrt%28%28x%2B9%2F7%29%5E2-x%5E2%29%2Fx&lang=ja
x=1/2