1977 AMC 12 Problem 28

Attempt Problem 28 of the 1977 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1977 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

28.

Let g(x)=x5+x4+x3g(x)=x^5+x^4+x^3 +x2+x+1.{}+x^2+x+1. What is the remainder when the polynomial g(x12)g(x^{12}) is divided by the polynomial g(x)?g(x)?
66
5−x5-x
4−x+x24-x+x^2
3−x+x2−x33-x+x^2-x^3
2−x+x2−x3+x42-x+x^2-x^3+x^4
Answer: A
Concepts:polynomialroots of unity
Small hint:
Use (x−1)g(x)=x6−1(x-1)g(x)=x^6-1
Big hint:
Evaluate the remainder at the five roots of gg and use its degree bound
Solution:
Let R(x)R(x) be the remainder, so deg⁡R≤4.\deg R\le4. The five roots α\alpha of gg satisfy α6=1\alpha^6=1 and α≠1.\alpha\ne1. Hence α12=1\alpha^{12}=1 and R(α)=g(α12)=g(1)=6. R(\alpha)=g(\alpha^{12})=g(1)=6. Therefore R(x)−6,R(x)-6, a polynomial of degree at most 4,4, has five distinct roots. It must be identically zero, so R(x)=6.R(x)=6.
Therefore, the correct answer is A.
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