The kissing circles theorem - challenging problem from Indonesia!

Thanks to Devan from Lilin Bangsa Intercultural School for suggesting this problem! What is the radius of the small circle in between the blue circle (radius 4) and green circle (radius 2)? This was a challenge problem for students aged 14 to 15. Thanks to all patrons! Special thanks to: Shrihari Puranik Kyle Professor X This is the only channel to feature math topics suggested by people around the world. Support the channel on Patreon so we can share the beauty of mathematics and make the world a better place: Websites consulted '_theorem Subscribe: Playlist to watch all videos on MindYourDecisions: