1991 AIME 第 4 题

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4.

有多少个实数 xx 满足方程 15log2x=sin(5πx)\frac15\log_2x=\sin(5\pi x)\text{?}

How many real numbers xx satisfy the equation 15log2x=sin(5πx)?\frac15\log_2x=\sin(5\pi x)?

答案:159
知识点:三角学对数交点计数
难度评级:2510
小提示:

由界限 sin(5πx)1\lvert\sin(5\pi x)\rvert\leq1,可将 xx 限制在一个有限区间内。

The bound sin(5πx)1\lvert\sin(5\pi x)\rvert\leq1 restricts xx to a finite interval

大提示:

分别考察正、负的正弦半波,并在每个符合条件的完整半波上数出两个交点。

Separate the positive and negative half-waves of the sine function and count two crossings on each eligible full half-wave

解答:

h(x)=sin(5πx)15log2xh(x)=\sin(5\pi x)-\frac15\log_2x。任何根都位于 [25,25]=[132,32][2^{-5},2^5]=[\frac{1}{32},32] 内。

x<1x<1 时,根只可能出现在正弦函数的负半波上。这样的半波有两个,即 (15,25)(\frac{1}{5},\frac{2}{5})(35,45)(\frac{3}{5},\frac{4}{5})。在每个半波的两个端点处,hh 都为正,而在中点处为负,因此各有两个根。在下降半段,单调性保证交点唯一;在上升半段,h=25π2sin(5πx)+15x2ln2>0\begin{aligned}h''&=-25\pi^2\sin(5\pi x)\\&\quad+\frac1{5x^2\ln2}>0\end{aligned}\text{,}所以 hh' 从中点处的负值开始递增,并且恰好一次等于零。因此,hh 先下降,再从中点的负值上升到端点的正值,所以这一半段恰有一个根。因此在 11 以下共有 44 个根。

此外,x=1x=1 也是一个根。当 1<x<321<x<32 时,根只可能出现在正半波 (2m5,2m+15)(\frac{2m}{5},\frac{2m+1}{5}) 上,其中 m=3m=344\ldots7979。这样的半波共有 7777 个。在每个半波的端点处,hh 为负,而在中点处为正。右半段严格递减。在左半段,h=125π3cos(5πx)25x3ln2<0\begin{aligned}h'''&=-125\pi^3\cos(5\pi x)\\&\quad-\frac2{5x^3\ln2}<0\end{aligned}\text{,}所以 hh' 是严格凹函数。它在左端点为正、在中点为负,因此恰好改变一次符号。于是 hh 先上升到唯一的极大值,再下降到仍为正的中点,所以左半段恰有一个交点。因此每个这样的半波恰好贡献两个根。故根的总数为 4+1+2(77)=1594+1+2(77)=159\text{。}

Let h(x)=sin(5πx)15log2x.h(x)=\sin(5\pi x)-\frac15\log_2x. Any root lies in [25,25]=[132,32].[2^{-5},2^5]=[\frac{1}{32},32].

For x<1,x<1, a root can occur only on a negative half-wave of the sine. There are two such half-waves, (15,25)(\frac{1}{5},\frac{2}{5}) and (35,45).(\frac{3}{5},\frac{4}{5}). On each, hh is positive at both endpoints and negative at the midpoint, so there are two roots. Uniqueness on the descending half follows from monotonicity; on the ascending half, h=25π2sin(5πx)+15x2ln2>0,\begin{aligned}h''&=-25\pi^2\sin(5\pi x)\\&\quad+\frac1{5x^2\ln2}>0,\end{aligned} so hh' increases from a negative value at the midpoint and vanishes once. The function hh therefore first decreases and then increases from a negative midpoint value to a positive endpoint value, giving exactly one root on this half. Thus there are 44 roots below 1.1.

Also x=1x=1 is a root. For 1<x<32,1<x<32, roots can occur only on positive half-waves (2m5,2m+15)(\frac{2m}{5},\frac{2m+1}{5}) with m=3,m=3, 4,4, ,\ldots, 79.79. There are 7777 of these. The value of hh is negative at each endpoint and positive at the midpoint. The right half is strictly decreasing. On the left half, h=125π3cos(5πx)25x3ln2<0,\begin{aligned}h'''&=-125\pi^3\cos(5\pi x)\\&\quad-\frac2{5x^3\ln2}<0,\end{aligned} so hh' is strictly concave. It is positive at the left endpoint and negative at the midpoint, so it changes sign exactly once. Consequently, hh rises to one maximum and then falls to a still-positive midpoint, giving exactly one crossing on the left half. Hence every such half-wave contributes exactly two roots. Therefore the total number is 4+1+2(77)=159.4+1+2(77)=159.

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