2025 AMC 12A 第 25 题

先试着解答 2025 AMC 12A 第 25 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2025 AMC 12A 解答,或核对答案

所有题目均经美国数学协会(MAA)官方合法授权使用。

25.

多项式 P(x)P(x)Q(x)Q(x) 都是 33 次,首项系数都是 11,并且它们的根都属于 {1,2,3,4,5}\{1, 2, 3, 4, 5\}。函数 f(x)=P(x)Q(x)f(x) = \dfrac{P(x)}{Q(x)} 具有如下性质:存在实数 a<b<c<da \lt b \lt c \lt d,使得所有满足 f(x)0f(x) \le 0 的实数 xx 的集合由闭区间 [a,b][a, b] 与开区间 (c,d)(c, d) 组成。可能有多少个函数 f(x)f(x)

Polynomials P(x)P(x) and Q(x)Q(x) each have degree 33 and leading coefficient 1,1, and their roots are all elements of {1,2,3,4,5}.\{1, 2, 3, 4, 5\}. The function f(x)=P(x)Q(x)f(x) = \dfrac{P(x)}{Q(x)} has the property that there exist real numbers a<b<c<da \lt b \lt c \lt d such that the set of all real numbers xx such that f(x)0f(x) \le 0 consists of the closed interval [a,b][a, b] together with the open interval (c,d).(c, d). How many functions f(x)f(x) are possible?

77

99

1111

1212

1313

答案:E
知识点:函数多项式分类讨论
难度评级:2540
解答:

本题已作废:按题面作答,正确答案不在选项中。下面给出计数。

对于 {f0}=[a,b](c,d),\{f \le 0\} = [a, b] \cup (c, d),闭区间端点 a,ba, b 必须是 PP 的零点,且在这些点 Q0,Q\ne0,c,dc,d 必须是极点。因此所需符号模式与 g(x)=(xa)(xb)(xc)(xd). g(x)=\frac{(x-a)(x-b)}{(x-c)(x-d)}. 相同。PP 中未使用的第三个因子必须与 QQ 中未使用的第三个因子相同;否则会多出零点、极点或符号变化。

如果公共因子是 a,b,c,d,a,b,c,d, 之外的第五个值,它不能在任一区间内部形成空点。在五个有序值中,它只能位于 a,a, 之前、bbc,c, 之间或 d,d, 之后,因此给出 33 个函数。

另一种可能是公共因子等于 ccd.d. 对每一种 (54)=5\binom54=5a,b,c,d,a,b,c,d, 的选择,这两个多项式对化简为同一个公式 gg,且定义域相同(都排除 c,dc,d),所以只定义一个函数。这再给出 55 个函数,按题面字面理解总数为 3+5=8.3+5=8.

如果在后五种情形中把两个多项式对分别计数,就会得到 3+25=13,3+2\cdot5=13,即临时答案 (E),但这没有回答题目所问的函数个数。因此印刷的选项都不正确。

This problem was voided: as written, its answer is not among the choices. Here is the count.

For {f0}=[a,b](c,d),\{f \le 0\} = [a, b] \cup (c, d), the endpoints a,ba, b of the closed interval must be zeros of PP at which Q0,Q\ne0, while c,dc,d must be poles. The required sign pattern is therefore that of g(x)=(xa)(xb)(xc)(xd). g(x)=\frac{(x-a)(x-b)}{(x-c)(x-d)}. The unused third factor of PP must match the unused third factor of Q;Q; otherwise there would be an extra zero, pole, or sign change.

If the common factor is the fifth value not among a,b,c,d,a,b,c,d, it cannot make a hole inside either interval. In the ordered list of five values it may therefore occur before a,a, between bb and c,c, or after d,d, giving 33 functions.

Alternatively, the common factor can equal cc or d.d. For each of the (54)=5\binom54=5 choices of a,b,c,d,a,b,c,d, these two polynomial pairs simplify to the same formula gg and have the same domain (both omit c,dc,d), so they define only one function. This gives 55 more functions, for a literal total of 3+5=8.3+5=8.

Counting the two polynomial pairs separately in each of the last five cases gives 3+25=13,3+2\cdot5=13, the provisional answer (E), but that does not answer the stated question about functions. Thus none of the printed choices is correct.

← 第 24 题#24
完整试卷

其他年份的第 25 题