在同一幅图中,画出方程为 y=∣x+1∣∣x−2∣y = \frac{|x + 1|}{|x - 2|} 和 y=ln⁡(1−x2)+12y = \ln\left(1 - \frac{x}{2}\right) + \tfrac{1}{2} 的曲线,指出所有渐近线的方程以及与坐标轴的交点坐标。清晰地标记这两条曲线。 由此求解不等式 ∣x+1∣∣x−2∣≥ln⁡(1−x2)+12\frac{|x + 1|}{|x - 2|} \ge \ln\left(1 - \frac{x}{2}\right) + \tfrac{1}{2}。

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完整解法

3(a)

1. Find the intercepts and asymptotes of the first curve $y = \frac{|x + 1|}{|x - 2|}$.

For C1:y=∣x+1∣∣x−2∣C_1: y = \frac{|x + 1|}{|x - 2|}:

  • yy-intercept: Let x=0x = 0, y=∣1∣∣−2∣=12y = \frac{|1|}{|-2|} = \frac{1}{2}. Point is (0,12)\left(0, \frac{1}{2}\right).
  • xx-intercept: Let y=0y = 0, ∣x+1∣=0  ⟹  x=−1|x + 1| = 0 \implies x = -1. Point is (−1,0)(-1, 0).
  • Vertical asymptote: The denominator is zero when x−2=0  ⟹  x=2x - 2 = 0 \implies x = 2.
  • Horizontal asymptote: lim⁡x→±∞∣x+1∣∣x−2∣=lim⁡x→±∞∣x+1x−2∣=1\lim_{x \to \pm\infty} \frac{|x + 1|}{|x - 2|} = \lim_{x \to \pm\infty} \left| \frac{x+1}{x-2} \right| = 1. Equation is y=1y = 1.

2. Find the intercepts and asymptotes of the second curve $y = \ln\left(1 - \frac{x}{2}\right) + \frac{1}{2}$.

For C2:y=ln⁡(1−x2)+12C_2: y = \ln\left(1 - \frac{x}{2}\right) + \frac{1}{2}:

  • yy-intercept: Let x=0x = 0, y=ln⁡(1)+12=12y = \ln(1) + \frac{1}{2} = \frac{1}{2}. Point is (0,12)\left(0, \frac{1}{2}\right).
  • xx-intercept: Let y=0y = 0,
    ln⁡(1−x2)+12=0\ln\left(1 - \frac{x}{2}\right) + \frac{1}{2} = 0
    ln⁡(1−x2)=−12\ln\left(1 - \frac{x}{2}\right) = -\frac{1}{2}
    1−x2=e−1/2  ⟹  x=2−2e−1/21 - \frac{x}{2} = e^{-1/2} \implies x = 2 - 2e^{-1/2}
    Point is (2−2e−1/2,0)\left(2 - 2e^{-1/2}, 0\right).
  • Vertical asymptote: The argument of the natural logarithm is zero when 1−x2=0  ⟹  x=21 - \frac{x}{2} = 0 \implies x = 2.

3. Sketch both curves on the same diagram, labelling all required features.

Using a graphing calculator as an aid or reasoning graphically:

  1. Draw the xx and yy axes.
  2. Draw the vertical line x=2x = 2 and horizontal line y=1y = 1 as dashed lines.
  3. Plot the axial intercepts (−1,0)(-1, 0), (2−2e−1/2,0)\left(2 - 2e^{-1/2}, 0\right), and the common yy-intercept (0,12)\left(0, \frac{1}{2}\right).
  4. Sketch C1C_1, noting the sharp "V-shape" minimum at (−1,0)(-1, 0) and its asymptotic behaviour towards x=2x = 2 and y=1y = 1. The curve is entirely in the region y≥0y \ge 0.
  5. Sketch C2C_2 for its domain x<2x < 2. The curve intersects the yy-axis at (0,12)\left(0, \frac{1}{2}\right) with a negative gradient, passes through its xx-intercept, and approaches the asymptote x=2x = 2 as y→−∞y \to -\infty.
  6. Label the respective curves and their key features clearly.
答案

Asymptotes: x=2x = 2 and y=1y = 1
Points of intersection with axes: (−1,0)(-1, 0), (2−2e−1/2,0)\left(2 - 2e^{-1/2}, 0\right), and (0,12)\left(0, \frac{1}{2}\right)

(A valid sketch must correctly illustrate both curves plotted on the same set of axes, with all asymptotes drawn as dashed lines and all intercepts labelled clearly.)

3(b)

1. Relate the inequality to the graphs from part (a) and state the point of intersection

Let y1=∣x+1∣∣x−2∣y_1 = \frac{|x + 1|}{|x - 2|} and y2=ln⁡(1−x2)+12y_2 = \ln\left(1 - \frac{x}{2}\right) + \tfrac{1}{2}.

The inequality y1≥y2y_1 \ge y_2 is satisfied where the curve of y1y_1 lies above or on the curve of y2y_2.
From the intercepts found in part (a), both curves have the same yy-intercept at (0,12)(0, \frac{1}{2}). Thus, they intersect at x=0x = 0.

2. Determine the solution set using the graphs and domain restrictions

From the sketched graphs, the curve of y1y_1 is above y2y_2 for x>0x > 0.
However, both functions share a vertical asymptote at x=2x = 2, and the logarithmic function y2y_2 is only defined for 1−x2>0  ⟹  x<21 - \frac{x}{2} > 0 \implies x < 2.
Therefore, the inequality holds for 0≤x<20 \le x < 2.

答案

0≤x<20 \le x < 2

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