Soddy circle

Inner and outer Soddy circles (red)
Picture credit: Wikipedia 
A solution to the threecircle form of the Apollonius
problem in which each of the given circles
is tangent to (just touches) the other two.^{1} There are two Soddy
circles: the outer Soddy circle, which surrounds the three
given circles, and the inner Soddy circle, which is interior
to them. The inner Soddy circle is the solution to the four
coins problem. Reference
 Soddy, F. "The Kiss Precise." Nature, 137: 1021, 1936.
Related category
• PLANE
CURVES 