1. (i)

    在同一坐标系中,画出方程为 y=ln⁡(x−2)y = \ln(x-2) 和 y=1ex+3y = \frac{1}{\text{e}^x} + 3 的曲线草图,并写出渐近线的方程。

  2. (ii)

    由此,解不等式 1ex+3≥ln⁡(x−2)\frac{1}{\text{e}^x} + 3 \ge \ln(x-2)。

  3. (iii)

    在不使用计算器的情况下,解不等式 1e∣x∣+3≥ln⁡(∣x∣−2)\frac{1}{\text{e}^{|x|}} + 3 \ge \ln(|x|-2)。

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完整解法

3(i)

1. Determine the features of $y = \ln(x-2)$ and its asymptote.

The curve y=ln⁡(x−2)y = \ln(x-2) is a translation of the standard logarithmic curve by 22 units to the right.
Domain: x−2>0  ⟹  x>2x - 2 > 0 \implies x > 2.
Vertical asymptote: x=2x = 2.
xx-intercept: Let y=0  ⟹  ln⁡(x−2)=0  ⟹  x−2=1  ⟹  x=3y = 0 \implies \ln(x-2) = 0 \implies x-2 = 1 \implies x = 3.
The xx-intercept is (3,0)(3,0).

2. Determine the features of $y = \frac{1}{\text{e}^x} + 3$ and its asymptote.

The curve y=1ex+3=e−x+3y = \frac{1}{\text{e}^x} + 3 = \text{e}^{-x} + 3 is a standard exponential decay curve shifted upwards by 33 units.
As x→∞x \to \infty, e−x→0\text{e}^{-x} \to 0, so y→3y \to 3. Hence, horizontal asymptote is y=3y = 3.
yy-intercept: Let x=0  ⟹  y=e0+3=4x = 0 \implies y = \text{e}^0 + 3 = 4.
The yy-intercept is (0,4)(0,4).

3. Describe the combined sketch.

Draw the axes, mark the intercepts (3,0)(3,0) and (0,4)(0,4), and draw dashed lines for the asymptotes x=2x=2 and y=3y=3.
Sketch the logarithmic curve approaching x=2x=2 downwards and growing towards the right.
Sketch the exponential curve decaying downwards and approaching y=3y=3 asymptotically towards the right.
(Both curves intersect in the first quadrant).

答案

Sketch showing:

  • Curve y=ln⁡(x−2)y = \ln(x-2) passing through (3,0)(3, 0) with vertical asymptote x=2x = 2.
  • Curve y=1ex+3y = \frac{1}{\text{e}^x} + 3 passing through (0,4)(0, 4) with horizontal asymptote y=3y = 3.

Equations of asymptotes: x=2,y=3x = 2, y = 3.

3(ii)

1. Find the $x$-coordinate of the point of intersection using a graphing calculator.

Using a graphing calculator to solve the equation 1ex+3=ln⁡(x−2)\frac{1}{\text{e}^x} + 3 = \ln(x-2):
x≈22.0855x \approx 22.0855

2. Determine the solution to the inequality using the visualised graphs and domain restrictions.

From the graphs sketched in part (i), the curve y=1ex+3y = \frac{1}{\text{e}^x} + 3 lies above or on the curve y=ln⁡(x−2)y = \ln(x-2) starting from the right of the vertical asymptote x=2x = 2 up to the point of intersection.

Therefore, the solution is 2<x≤22.08552 < x \le 22.0855.
Rounding to 3 significant figures, we get:
2<x≤22.12 < x \le 22.1

答案

2<x≤22.12 < x \le 22.1

3(iii)

1. State the solution to part (ii) to use as a foundation.

Since part (iii) depends on (ii), we first evaluate the solution to (ii).
Let f(x)=1ex+3f(x) = \frac{1}{\text{e}^x} + 3 and g(x)=ln⁡(x−2)g(x) = \ln(x-2).
Using a graphing calculator, the intersection of y=f(x)y=f(x) and y=g(x)y=g(x) is at x=α≈22.086x = \alpha \approx 22.086.
From the graphs sketched in (i), f(x)≥g(x)f(x) \ge g(x) between the vertical asymptote x=2x = 2 and the intersection point.
Thus, the solution to (ii) is 2<x≤α2 < x \le \alpha.

2. Relate the inequality in (iii) to the result of (ii) using substitution.

The inequality 1e∣x∣+3≥ln⁡(∣x∣−2)\frac{1}{\text{e}^{|x|}} + 3 \ge \ln(|x|-2) is identical to the inequality in (ii), but with xx replaced by ∣x∣|x|.
Substituting ∣x∣|x| into the solution from (ii) gives:
2<∣x∣≤α2 < |x| \le \alpha

3. Solve the resulting modulus inequality.

The inequality 2<∣x∣≤α2 < |x| \le \alpha splits into two regions:
When x>0x > 0, we have 2<x≤α2 < x \le \alpha.
When x<0x < 0, we have 2<−x≤α  ⟹  −α≤x<−22 < -x \le \alpha \implies -\alpha \le x < -2.

Combining these and substituting α≈22.086\alpha \approx 22.086, we get:
−22.086≤x<−2 or 2<x≤22.086-22.086 \le x < -2 \text{ or } 2 < x \le 22.086

Rounding to 3 significant figures, the final solution is:
−22.1≤x<−2 or 2<x≤22.1-22.1 \le x < -2 \text{ or } 2 < x \le 22.1

答案

−22.1≤x<−2 or 2<x≤22.1-22.1 \le x < -2 \text{ or } 2 < x \le 22.1

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