Maths Olympiad Prep

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Algebra Difficulty 5.4 AIME, harder Find the answer

Let n>0n>0 be an integer. Each face of a regular tetrahedron is painted in one of nn colors (the faces are not necessarily painted different colors.) Suppose there are n3n^{3} possible colorings, where rotations, but not reflections, of the same coloring are considered the same. Find all possible values of nn.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We count the possible number of colorings. If four colors are used, there are two different colorings that are mirror images of each other, for a total of 2(n4)2\binom{n}{4} colorings. If three colors are used, we choose one color to use twice (which determines the coloring), for a total of 3(n3)3\binom{n}{3} colorings. If two colors are used, we can either choose one of those colors and color three faces with it, or we can color two faces each color, for a total of 3(n2)3\binom{n}{2} colorings. Finally, we can also use only one color, for (n1)\binom{n}{1} colorings. This gives a total of 2(n4)+3(n3)+3(n2)+(n1)=112n2(n2+11)2\binom{n}{4}+3\binom{n}{3}+3\binom{n}{2}+\binom{n}{1}=\frac{1}{12} n^{2}\left(n^{2}+11\right) colorings. Setting this equal to n3n^{3}, we get the equation n2(n2+11)=12n3n^{2}\left(n^{2}+11\right)=12 n^{3}, or equivalently n2(n1)(n11)=0n^{2}(n-1)(n-11)=0, giving the answers 1 and 11.

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