2020 AMC 12B 第 17 题
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17.
有多少个形如 的多项式,其中 和 都是实数,满足如下性质:只要 是一个根,那么 也是一个根?(注:。)
How many polynomials of the form where and are real numbers, have the property that whenever is a root, so is (Note that )
答案:C
解答:
设 ,它是本原三次单位根。由于 不是根,不同根的集合在乘以 后保持不变,所以根按 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 之和为 。
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 或 。
全部根的乘积必须为 。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 ;取 即可。共轭对称的重数方案为 和 ,因此有 个多项式。
所以正确答案是 C。
Here is a primitive cube root of unity. Since is not a root, the set of distinct roots is closed under multiplication by so it consists of triples equally spaced in argument. Five roots cannot fill two such triples, so there is exactly one triple, with multiplicities summing to
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 and
The product of the roots must equal 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 setting works, and the two conjugate-symmetric multiplicity patterns and each give a valid polynomial. Hence there are
Thus, the correct answer is C.
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