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2022 AMC 12A Problem 21

Problem 21 of 25HarderAlgebra

Let P(x)=x2022+x1011+1.P(x)=x^{2022}+x^{1011}+1. Which of the following polynomials divides P(x)?P(x)?

Answer choices

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Solution

If ζ\zeta is a root of a divisor, then P(ζ)=ζ2022+ζ1011+1=0P(\zeta)=\zeta^{2022}+\zeta^{1011}+1=0 is required. The roots of x6+x3+1x^6+x^3+1 are the primitive 99th roots of unity, so ζ9=1.\zeta^9=1. Reducing exponents modulo 9,9, 202262022\equiv6 and 10113,1011\equiv3, giving ζ6+ζ3+1.\zeta^6+\zeta^3+1. Here ω=ζ3\omega=\zeta^3 is a primitive cube root of unity, so this equals ω2+ω+1=0.\omega^2+\omega+1=0. The other four options fail: substituting their roots yields nonzero values (for instance, the primitive cube roots of unity give P=3P=3). Thus, the correct answer is E.

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Concepts: roots of unity · polynomial

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.