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*‘|.