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CIE 9709 2023 June Paper 31 Q2

A Level / CIE / P3

CIE 9709 2023 June Paper 31 Paper · Question 2

题目

Problem

(a) Sketch the graph of y=∣2x+3∣y=|2x+3|.

(1)

(b) Solve the inequality 3x+8>∣2x+3∣3x+8>|2x+3|.

(3)
题目中文翻译

(a) 画出 y=∣2x+3∣y=|2x+3| 的图像草图。

(1)

(b) 解不等式 3x+8>∣2x+3∣3x+8>|2x+3|。

(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

(a)

解法一

思路

展开

将直线 y=2x+3y=2x+3 在横轴下方的部分向上反射,得到 V 形图。

答题过程

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The graph consists of the two rays

y={−2x−3,x<−32,2x+3,x≥−32.y= \begin{cases} -2x-3,&x<-\tfrac32,\\ 2x+3,&x\ge-\tfrac32. \end{cases}

The vertex is (−3/2,0)(-3/2,0) and the yy-intercept is (0,3)(0,3). The branches have gradients −2-2 and 22.

(b)

解法一

思路

展开

绝对值小于某个数等价于该数同时大于绝对值内表达式及其相反数。这种写法自动排除右侧为负的错误范围。

答题过程

展开

For 3x+8>∣2x+3∣3x+8>|2x+3|, both

3x+8> 2x+3,3x+8> −(2x+3)\begin{align*} 3x+8>&\,2x+3,\\[4mm] 3x+8>&\,-(2x+3) \end{align*}

must hold. These give

x> −5,5x> −11.\begin{align*} x>&\,-5,\\[4mm] 5x>&\,-11. \end{align*}

Taking the intersection,

x>−115.\boxed{x>-\frac{11}{5}}.

解法二

思路

展开

平方前必须先要求 3x+8>03x+8>0。平方后的二次不等式会产生额外的负区间,需与该符号条件相交。

答题过程

展开

Since ∣2x+3∣≥0|2x+3|\ge0, the inequality requires

3x+8>0,sox>−83.3x+8>0,\quad\text{so}\quad x>-\frac83.

Under this condition, squaring is equivalent:

(3x+8)2> (2x+3)2,5x2+36x+55> 0,(x+5)(5x+11)> 0.\begin{align*} (3x+8)^2>&\,(2x+3)^2,\\[4mm] 5x^2+36x+55>&\,0,\\[4mm] (x+5)(5x+11)>&\,0. \end{align*}

Thus x<−5x<-5 or x>−11/5x>-11/5. Intersecting with x>−8/3x>-8/3 leaves

x>−115.\boxed{x>-\frac{11}{5}}.