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CIE 9709 2023 June Paper 32 Q2

A Level / CIE / P3

CIE 9709 2023 June Paper 32 Paper · Question 2

题目

Problem

Solve the equation ln⁡(2x2−3)=2ln⁡x−ln⁡2\ln(2x^2-3)=2\ln x-\ln2, giving your answer in an exact form.

(3)
题目中文翻译

解方程 ln⁡(2x2−3)=2ln⁡x−ln⁡2\ln(2x^2-3)=2\ln x-\ln2,答案用精确形式表示。

(3)
原卷通用作答要求

Answer all questions. You should use a calculator where appropriate.

You must show all necessary working clearly; no marks will be given for unsupported answers from a calculator.

Give non-exact numerical answers correct to 3 significant figures, or 1 decimal place for angles in degrees, unless a different level of accuracy is specified in the question.

请回答所有题目。适当时应使用计算器。必须清楚展示所有必要的计算过程;仅给出计算器答案而没有过程,不予评分。除非题目另有精度要求,非精确数值答案保留 3 位有效数字,以度为单位的角度保留 1 位小数。

解答

解法一

思路

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先检查对数的定义域,再用对数法则合并右侧。解出的负根会违反 ln⁡x\ln x 的定义域。

答题过程

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The logarithms require x>0x>0 and 2x2−3>02x^2-3>0, hence x>3/2x>\sqrt{3/2}.

Using logarithm laws,

2ln⁡x−ln⁡2=ln⁡x22.2\ln x-\ln2=\ln\frac{x^2}{2}.

Therefore

2x2−3= x22,3x2= 6,x2= 2.\begin{align*} 2x^2-3=&\,\frac{x^2}{2},\\[4mm] 3x^2=&\,6,\\[4mm] x^2=&\,2. \end{align*}

The negative root is outside the domain, while x=2x=\sqrt2 satisfies it. Hence x=2\boxed{x=\sqrt2}.