Circles that intersect at only one point.
Properties
- The distance between the centres O and O’ of two internally tangent circles is equal to the difference of their radii : d(O, O’) = |r – r‘|.
- The distance between the centres O and O’ of two externally tangent circles is equal to the sum of their radii : d(O, O’) = |r – r‘|.
Examples
Externally tangent circles : 
Internally tangent circles :
