知识点1 圆与圆的位置关系
设两圆圆心分别为$O_{1},O_{2}$,半径分别为$r_{1}、r_{2}$,|$O_{1}O_{2}$|=$d$
(1)$d > r_{1}+r_{2}$⇔外离;
(2)$d = r_{1}+r_{2}$⇔外切;
(3)|$r_{1}-r_{2}$| <$d$<|$r_{1}+r_{2}$|⇔相交;
(4)$d$=|$r_{1}-r_{2}$|⇔内切;
(5)0 <$d$<|$r_{1}-r_{2}$|⇔内含.
知识点2 圆与圆的公切线条数
(1)外离⇔4条公切线
(2)外切⇔3条公切线
(3)相交⇔2条公切线
(4)内切⇔1条公切线
(5)内含⇔0条公切线