。
C1: x^2+y^2+2x+2y-2=0(x+1)^2+(y+1)^2 = 4center O1(-1, 1) , r1=2C2:x^2+y^2-4x-2y+4=0(x-2)^2+(y-1)^2 =1center O2(2,1) , r2=1|O1O2|^2 = (-1-2)^2 +(1-1)^2 = 9|O1O2| =3=r1+r2公共切线有 ： 3条
