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.

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Websites consulted
https://en.wikipedia.org/wiki/Descartes%27_theorem
https://www.cut-the-knot.org/p....ythagoras/TangentCir
http://lostmathlessons.blogspo....t.com/2015/12/kissin

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