Illustration of all eight solutions to Apollonius' problem.  The three given circles are labeled C1, C2 and C3 and are colored red, green and blue, respectively.  The solutions are grouped into four pairs, one pink and one black solution circle, and labeled as 1A/1B, 2A/2B, 3A/3B, and 4A/4B.
<a href="//commons.wikimedia.org/wiki/User:WillowW" title="User:WillowW">WillowW</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>