Difference between revisions of "2021 WSMO Accuracy Round Problems/Problem 8"

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Latest revision as of 12:58, 14 December 2023

Problem

20 unit spheres are stacked in a triangular pyramid formation, such that the first layer has 1 sphere, the second layer has 3 spheres, the third layer has 6 spheres, and the fourth layer has 10 spheres. The radius of the smallest sphere that fully contains all of these spheres is $\frac{a\sqrt{b}+c}{d},$ where $\gcd{(a,c,d)}=1$ and $b$ is not divisible by the square of any prime. Find $a+b+c+d.$

Solution