已知 aa 和 bb 都是正数常数,在同一幅图上,草拟以下函数的图像:
y=2∣x−a∣andy=b∣x−a∣.y = \frac{2}{|x - a|} \quad \text{and} \quad y = b|x - a|.
由此或通过其他方法,解不等式 2∣x−a∣>b∣x−a∣\frac{2}{|x - a|} > b|x - a|。

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完整解法

7(a)

1. Determine the shape and key features of $y = \frac{2}{|x - a|}$.

The expression ∣x−a∣|x - a| is always positive for x≠ax \neq a, so y=2∣x−a∣y = \frac{2}{|x - a|} is strictly positive.
As x→ax \to a, the denominator approaches 00, so y→∞y \to \infty, meaning x=ax = a is a vertical asymptote.
As x→±∞x \to \pm\infty, y→0y \to 0, meaning the xx-axis (y=0y = 0) is a horizontal asymptote.
At x=0x = 0, the yy-intercept is y=2∣−a∣=2ay = \frac{2}{|-a|} = \frac{2}{a} (since a>0a > 0).

2. Determine the shape and key features of $y = b|x - a|$.

The function y=b∣x−a∣y = b|x - a| is a modulus graph, which forms a 'V' shape opening upwards (since b>0b > 0).
The minimum point (vertex) occurs when ∣x−a∣=0|x - a| = 0, giving the coordinate (a,0)(a, 0).
At x=0x = 0, the yy-intercept is y=b∣−a∣=aby = b|-a| = ab.

3. Sketch both graphs on the same coordinate axes.

Draw the xx and yy axes.
Mark x=ax = a on the positive xx-axis and draw a vertical dotted line to represent the asymptote and axis of symmetry.
Plot the vertex (a,0)(a, 0) and draw the straight V-shape lines for y=b∣x−a∣y = b|x - a|, labelling the yy-intercept abab.
Draw the two symmetric branches of y=2∣x−a∣y = \frac{2}{|x - a|} approaching x=ax = a upwards and the xx-axis outwards, labelling the yy-intercept 2a\frac{2}{a}.
Ensure the graphs intersect exactly twice, once on each side of x=ax = a.

答案

A single sketch containing:

  • A vertical asymptote labelled x=ax = a.
  • The V-shaped graph y=b∣x−a∣y = b|x - a| with vertex at (a,0)(a, 0) and yy-intercept (0,ab)(0, ab).
  • The two-branch curve y=2∣x−a∣y = \frac{2}{|x - a|} in the upper quadrants, approaching x=ax = a and y=0y = 0, with yy-intercept (0,2a)(0, \frac{2}{a}).
  • Both graphs intersecting at two points, one on each side of x=ax = a.

7(b)

1. Multiply both sides of the inequality by $|x - a|$ to eliminate the denominator, noting the condition for validity.

For the expression 2∣x−a∣\frac{2}{|x - a|} to be defined, x≠ax \neq a.
For x≠ax \neq a, we know that ∣x−a∣>0|x - a| > 0.
Since ∣x−a∣|x - a| is strictly positive, we can multiply both sides of the inequality by ∣x−a∣|x - a| without reversing the inequality sign:
2∣x−a∣>b∣x−a∣\frac{2}{|x - a|} > b|x - a|
2>b∣x−a∣22 > b|x - a|^2

2. Solve the resulting inequality for $x - a$.

Since bb is a positive constant (b>0b > 0), we divide by bb:
∣x−a∣2<2b|x - a|^2 < \frac{2}{b}
Since ∣x−a∣2=(x−a)2|x - a|^2 = (x - a)^2 and the square root of both sides gives ∣x−a∣|x - a|:
∣x−a∣<2b|x - a| < \sqrt{\frac{2}{b}}
Using the property of modulus inequalities (∣X∣<k  ⟺  −k<X<k|X| < k \iff -k < X < k):
−2b<x−a<2b-\sqrt{\frac{2}{b}} < x - a < \sqrt{\frac{2}{b}}
a−2b<x<a+2ba - \sqrt{\frac{2}{b}} < x < a + \sqrt{\frac{2}{b}}

3. State the final solution set, excluding the value where the original expression is undefined.

The original inequality is undefined at x=ax = a.
Therefore, we must explicitly exclude x=ax = a from the solution interval.
a−2b<x<aora<x<a+2ba - \sqrt{\frac{2}{b}} < x < a \quad \text{or} \quad a < x < a + \sqrt{\frac{2}{b}}

答案

a−2b<x<aa - \sqrt{\frac{2}{b}} < x < a or a<x<a+2ba < x < a + \sqrt{\frac{2}{b}}

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