直线方程代入,得：x^2+(2x+1)^2=r^2 ,5x^2+4x+1-r^2=0(x1-x2)^2=[4^2-4*5*(1-r^2)]/5^2=(20r^2-4)/25根据直线斜率为2,∴ (y1-y2)^2=4 (x1-x2)^2∴(20r^2-4)/25×（1+4）=5^2解得：r=√645/10