2020 AMC 12B 第 17 题

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17.

有多少个形如 x5+ax4+bx3+cx2+dxx^5 + ax^4 + bx^3 + cx^2 + dx +2020+ 2020 的多项式,其中 aabbccdd 都是实数,满足如下性质:只要 rr 是一个根,那么 1+i32r\dfrac{-1 + i\sqrt3}{2}\cdot r 也是一个根?(注:i=1i = \sqrt{-1}。)

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

答案:C
知识点:单位根复数多项式
难度评级:1960
小提示:

ω=1+i32\omega = \dfrac{-1 + i\sqrt3}{2} 是本原三次单位根,所以非零根按 {r,ωr,ω2r}\{r, \omega r, \omega^2 r\} 这样的三元组出现。

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\}

大提示:

55 个根只能来自一个不同根三元组并带有重数;实系数还要求根关于共轭对称。

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

解答:

ω=1+i32\omega = \tfrac{-1 + i\sqrt3}{2},它是本原三次单位根。由于 00 不是根,不同根的集合在乘以 ω\omega 后保持不变,所以根按 {r,ωr,ω2r}\{r, \omega r, \omega^2 r\} 这样的三元组出现。五个根无法填满两个三元组,因此只有一个三元组,其重数 m1,m2,m31m_1, m_2, m_3 \ge 1 之和为 55

实系数要求根的多重集在共轭下不变。这只可能发生在三元组的辐角关于实轴对称时,即 {0,120,240}\{0^\circ, 120^\circ, 240^\circ\}{60,180,300}\{60^\circ, 180^\circ, 300^\circ\}

全部根的乘积必须为 2020-2020。第一种构型中的实根为正,会导致乘积为正,不可能。第二种构型中的实根为负,乘积为 ρ5-\rho^5;取 ρ5=2020\rho^5 = 2020 即可。共轭对称的重数方案为 (1,3,1)(1, 3, 1)(2,1,2)(2, 1, 2),因此有 22 个多项式。

所以正确答案是 C

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.

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