圆CIC2的共切线方程
x^2+y^2+2x+6y++9+k(x^2+y^2-6x+2y+1)=0
得
(1+k)x^2+(1+k)y^2+(2-6k)x+(6+2k)y+(9+k)=0
显然 1+k=0
k=-1
圆CIC2的共切线方程
2x+y+2=0