2020 AMC 12B Problem 17

Attempt Problem 17 of the 2020 AMC 12B 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 2020 AMC 12B solutions, or check the answer key.

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17.

How many polynomials of the form x5+ax4+bx3+cx2+dxx^5 + ax^4 + bx^3 + cx^2 + dx +2020,+ 2020, where a,a, b,b, c,c, and dd are real numbers, have the property that whenever rr is a root, so is 1+i32r?\dfrac{-1 + i\sqrt3}{2}\cdot r? (Note that i=1.i = \sqrt{-1}.)

00

11

22

33

44

Answer: C
Concepts:roots of unitycomplex numberpolynomial
Difficulty rating: 1960
Small Hint:

ω=1+i32\omega = \dfrac{-1 + i\sqrt3}{2} is a primitive cube root of unity, so nonzero roots come in triples {r,ωr,ω2r}\{r, \omega r, \omega^2 r\}

Big Hint:

With five roots the distinct roots form one triple with multiplicities summing to 5;5; real coefficients force conjugate symmetry

Solution:

Here ω=1+i32\omega = \tfrac{-1 + i\sqrt3}{2} is a primitive cube root of unity. Since 00 is not a root, the set of distinct roots is closed under multiplication by ω,\omega, so it consists of triples {r,ωr,ω2r}\{r, \omega r, \omega^2 r\} equally spaced in argument. Five roots cannot fill two such triples, so there is exactly one triple, with multiplicities m1,m2,m31m_1, m_2, m_3 \ge 1 summing to 5.5.

Real coefficients require the root multiset to be closed under conjugation. This is possible only when the triple’s arguments are symmetric about the real axis, which happens for the two configurations {0,120,240}\{0^\circ, 120^\circ, 240^\circ\} and {60,180,300}.\{60^\circ, 180^\circ, 300^\circ\}.

The product of the roots must equal 2020.-2020. In the first configuration the real root is positive, forcing a positive product, which is impossible. In the second, the real root is negative and the product is ρ5;-\rho^5; setting ρ5=2020\rho^5 = 2020 works, and the two conjugate-symmetric multiplicity patterns (1,3,1)(1, 3, 1) and (2,1,2)(2, 1, 2) each give a valid polynomial. Hence there are 2.2.

Thus, the correct answer is C.

Problem 16#16
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