S大圆=Pi*R²
S小圆=Pi*r²
(S大圆-S小圆)/S大圆=2/3得:
(Pi*R²-Pi*r²)/Pi*R² =2/3
(R²-r²)/R²=2/3
1-r²/R²=2/3
r²/R²=1/3
R/r=根号3
大圆的半径R与小圆的半径r之比是根号3