Three discs are mutually tangent among themselves and with an enclosing circle of radius 18. The blue circle has radius 9. What it the common value of the radius of the gray discs?

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## Solution

The gray radii area 8.

## One reply on “The tritium nucleus”

It is somewhat of a mystery to me why you can only fit one extra radii 9 circle, but you can fit two whole radii 8 circles. Are there other surprises of this sort?