题目
Problem
In this question you must show all stages of your working.
Solutions relying entirely on calculator technology are not acceptable.
Use algebra to determine the set of values of x for which
∣x+8∣x2−9>6−2x
(6)
解答
解法一
思路
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因为分母是 ∣x+8∣,所以要以 x=−8 为分界点分情况讨论。注意 x=−8 本身不能取。
在每个区间内把绝对值去掉,再解普通二次不等式。
答题过程
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We need
∣x+8∣x2−9>6−2x,
where
x=−8.
If x>−8, then ∣x+8∣=x+8. Since x+8>0,
x2−9x2−9x2−93x2+10x−57>(x+8)(6−2x)>6x−2x2+48−16x>−2x2−10x+48>0.
Factorise:
3x2+10x−57=(3x+19)(x−3).
So, in this case,
(3x+19)(x−3)>0.
This gives
x<−319orx>3.
Combining with x>−8 gives
−8<x<−319orx>3.
If x<−8, then ∣x+8∣=−(x+8). Since ∣x+8∣>0, multiply by ∣x+8∣:
x2−9x2−9>−(x+8)(6−2x)>(−x−8)(6−2x).
Compute the right hand side:
(−x−8)(6−2x)==−6x+2x2−48+16x2x2+10x−48.
Therefore
x2−90x2+10x−39>2x2+10x−48>x2+10x−39<0.
Factorise:
x2+10x−39=(x+13)(x−3).
Thus
(x+13)(x−3)<0,
so
−13<x<3.
Combining with x<−8 gives
−13<x<−8.
Therefore the full solution is
−13<x<−8or−8<x<−319orx>3.
解法二
思路
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另一种思路是先找出所有临界点(即等式成立的边界值以及无定义点),然后在数轴上划分区间进行代值测试。此法可免去在不等式中处理负号和不等号方向变号的困扰。
等式方程为 ∣x+8∣x2−9=6−2x,因为绝对值恒为非负,故可直接变形为 x2−9=(6−2x)∣x+8∣,再依 x>−8 和 x<−8 分类找出所有解。
答题过程
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First, identify all critical values. The vertical asymptote (where the expression is undefined) is:
x=−8
Now solve the equation for the boundary points of intersection:
∣x+8∣x2−9=6−2x
Since ∣x+8∣>0 for all x=−8, this is equivalent to:
x2−9=(6−2x)∣x+8∣
Case 1: x>−8, which means ∣x+8∣=x+8:
x2−9=x2−9=3x2+10x−57=(3x+19)(x−3)=x=−319(6−2x)(x+8)−2x2−10x+4800orx=3
Both values are greater than −8, so they are valid critical values.
Case 2: x<−8, which means ∣x+8∣=−(x+8):
x2−9=x2−9=x2+10x−39=(x+13)(x−3)=x=−13−(6−2x)(x+8)2x2+10x−4800orx=3
Since we require x<−8, the only valid root in this case is:
x=−13
Thus, the critical values are:
x=−13,x=−8,x=−319,x=3
These values divide the real number line into five intervals. Testing a value from each interval in the inequality ∣x+8∣x2−9>6−2x yields:
- For x<−13: e.g., x=−14⟹6187≈31.17>34 (False)
- For −13<x<−8: e.g., x=−10⟹291=45.5>26 (True)
- For −8<x<−319: e.g., x=−7⟹40>20 (True)
- For −319<x<3: e.g., x=0⟹−89>6 (False)
- For x>3: e.g., x=4⟹127≈0.58>−2 (True)
Therefore, the inequality holds on:
−13<x<−8or−8<x<−319orx>3