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/Descart...
https://www.cut-the-knot.org/pythagor...
http://lostmathlessons.blogspot.com/2...

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My Books
"The Joy of Game Theory" shows how you can use math to out-think your competition. (rated 3.9/5 stars on 35 reviews)
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"The Irrationality Illusion: How To Make Smart Decisions And Overcome Bias" is a handbook that explains the many ways we are biased about decision-making and offers techniques to make smart decisions. (rated 4.6/5 stars on 3 reviews)
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"Math Puzzles Volume 1" features classic brain teasers and riddles with complete solutions for problems in counting, geometry, probability, and game theory. Volume 1 is rated 4.4/5 stars on 13 reviews.
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"Math Puzzles Volume 2" is a sequel book with more great problems. (rated 4.5/5 stars on 6 reviews)
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"Math Puzzles Volume 3" is the third in the series. (rated 4/5 stars on 6 reviews)
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"40 Paradoxes in Logic, Probability, and Game Theory" contains thought-provoking and counter-intuitive results. (rated 4.4/5 stars on 13 reviews)
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"The Best Mental Math Tricks" teaches how you can look like a math genius by solving problems in your head (rated 4.8/5 stars on 5 reviews)
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"Multiply Numbers By Drawing Lines" This book is a reference guide for my video that has over 1 million views on a geometric method to multiply numbers. (rated 4.4/5 stars on 5 reviews)
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