Skip to content
CalcGospel 國際數學圖譜
返回

P2 2022 May Q8

A Level / Edexcel / P2

IAL 2022 May Paper · Question 8

题目

Problem

In this question you must show all stages of your working.

Solutions relying entirely on calculator technology are not acceptable.

A curve has equation

y=256x4−304x−35+27x2x≠0y = 256x^4 - 304x - 35 + \frac{27}{x^2} \quad x \neq 0

(a) Find dydx\frac{\mathrm{d}y}{\mathrm{d}x}

(3)

(b) Hence find the coordinates of the stationary points of the curve.

(5)
题目中文翻译

在本题中,你必须展示所有的计算步骤。纯依赖计算器技术求解的方案是不可接受的。

一条曲线的方程为:

y=256x4−304x−35+27x2x≠0y = 256x^4 - 304x - 35 + \frac{27}{x^2} \quad x \neq 0

(a) 求导数 dydx\frac{\mathrm{d}y}{\mathrm{d}x}。 (3)

(b) 由此求出该曲线所有驻点(Stationary points)的坐标。 (5)

解答

(a)

解法一

思路

展开

为了求导,我们首先需要将曲线方程中的分式项写成指数幂的形式:

27x2=27x−2\frac{27}{x^2} = 27x^{-2}

因此曲线方程可以重写为:

y=256x4−304x−35+27x−2y = 256x^4 - 304x - 35 + 27x^{-2}

根据导数法则:

  • ddx(xn)=nxn−1\frac{\mathrm{d}}{\mathrm{d}x}(x^n) = nx^{n-1}
  • 对于常数项,其导数为 00

逐项对 yy 求导即可。

答题过程

展开

Rewrite the curve equation by expressing the last term as a power of xx:

y=256x4−304x−35+27x−2y = 256x^4 - 304x - 35 + 27x^{-2}

Differentiate with respect to xx term-by-term:

dydx= 256(4x3)−304(1)−0+27(−2x−3)= 1024x3−304−54x−3= 1024x3−304−54x3\begin{align*} \frac{\mathrm{d}y}{\mathrm{d}x} =&\, 256(4x^3) - 304(1) - 0 + 27(-2x^{-3}) \\ =&\, 1024x^3 - 304 - 54x^{-3} \\ =&\, 1024x^3 - 304 - \frac{54}{x^3} \end{align*}

(b)

解法一:换元法求解一元二次方程

思路

展开
  1. 设定驻点条件: 在驻点处,曲线的斜率为 00,即令导数等于 00:
1024x3−304−54x3=01024x^3 - 304 - \frac{54}{x^3} = 0
  1. 化简方程为多项式形式: 在方程两边同乘以 x3x^3 (因为 x≠0x \neq 0):
1024(x3)2−304(x3)−54=01024(x^3)^2 - 304(x^3) - 54 = 0

同除以公约数 22 以简化方程:

512(x3)2−152(x3)−27=0512(x^3)^2 - 152(x^3) - 27 = 0
  1. 利用换元法解关于 x3x^3 的方程: 令 u=x3u = x^3,上面的方程可写作一个关于 uu 的一元二次方程:
512u2−152u−27=0512u^2 - 152u - 27 = 0

将此方程因式分解或利用求根公式求出 uu 的两个根:

(8u+1)(64u−27)=0  ⟹  u=−18或u=2764(8u + 1)(64u - 27) = 0 \implies u = -\frac{1}{8} \quad \text{或} \quad u = \frac{27}{64}
  1. 求出自变量 xx 的值: 由于 u=x3u = x^3,对两个根分别进行开立方根(Cube root)计算:

    • 若 x3=−18  ⟹  x=−12x^3 = -\frac{1}{8} \implies x = -\frac{1}{2}
    • 若 x3=2764  ⟹  x=34x^3 = \frac{27}{64} \implies x = \frac{3}{4}
  2. 代回原曲线方程求解 yy 坐标:

    • 将 x=−12x = -\frac{1}{2} 代入原曲线方程中计算 yy。
    • 将 x=34x = \frac{3}{4} 代入原曲线方程中计算 yy。 最终写出所有驻点坐标。

答题过程

展开

At the stationary points, the gradient is zero:

dydx=0  ⟹  1024x3−304−54x3=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0 \implies 1024x^3 - 304 - \frac{54}{x^3} = 0

Since x≠0x \neq 0, multiply the entire equation by x3x^3:

1024(x3)2−304(x3)−54=01024(x^3)^2 - 304(x^3) - 54 = 0

Divide by 22:

512(x3)2−152(x3)−27=0512(x^3)^2 - 152(x^3) - 27 = 0

Let u=x3u = x^3. The equation becomes:

512u2−152u−27=0512u^2 - 152u - 27 = 0

Factorize the quadratic expression:

(8u+1)(64u−27)=0(8u + 1)(64u - 27) = 0

This gives two possible values for uu:

u=−18oru=2764u = -\frac{1}{8} \quad \text{or} \quad u = \frac{27}{64}

Substitute back u=x3u = x^3 and take the cube root:

  • Case 1:
x3=−18  ⟹  x=−12x^3 = -\frac{1}{8} \implies x = -\frac{1}{2}
  • Case 2:
x3=2764  ⟹  x=34x^3 = \frac{27}{64} \implies x = \frac{3}{4}

Now, find the corresponding yy-coordinates by substituting xx back into the original curve equation:

  • For x=−12x = -\frac{1}{2}:
y= 256(−12)4−304(−12) −35+27(−1/2)2= 256(116)+152−35+2714= 16+152−35+108= 241\begin{align*} y =&\,256\Big(-\frac12\Big)^4-304\Big(-\frac12\Big)\\ &\,\hspace{2pt}-35+\frac{27}{(-1/2)^2}\\ =&\, 256\Big(\frac{1}{16}\Big) + 152 - 35 + \frac{27}{\frac{1}{4}} \\ =&\, 16 + 152 - 35 + 108 \\ =&\, 241 \end{align*}
  • For x=34x = \frac{3}{4}:
y= 256(34)4−304(34) −35+27(3/4)2= 256(81256)−228−35+27916= 81−228−35+27×169= 81−228−35+48= −134\begin{align*} y =&\,256\Big(\frac34\Big)^4-304\Big(\frac34\Big)\\ &\,\hspace{2pt}-35+\frac{27}{(3/4)^2}\\ =&\, 256\Big(\frac{81}{256}\Big) - 228 - 35 + \frac{27}{\frac{9}{16}} \\ =&\, 81 - 228 - 35 + 27 \times \frac{16}{9} \\ =&\, 81 - 228 - 35 + 48 \\ =&\, -134 \end{align*}

The coordinates of the stationary points are:

(−12,241)and(34,−134)\Big( -\frac{1}{2}, 241 \Big) \quad \text{and} \quad \Big( \frac{3}{4}, -134 \Big)