图中展示了 y=f(∣x∣)y = f(|x|) 和 y=f′(x)y = f'(x) 的图像。 y=f(∣x∣)y = f(|x|) 的图像在 (−3,4)(-3,4) 和 (3,4)(3,4) 处有转折点,与 xx 轴相交于 (−2,0)(-2,0) 和 (2,0)(2,0),其渐近线方程为 x=−1x = -1、x=1x = 1 和 y=2y = 2。y=f′(x)y = f'(x) 的图像与 xx 轴相交于 (3,0)(3,0),其渐近线方程为 x=1x = 1 和 y=0y = 0。 在不同的图上分别画出下列函数的图像,并清晰地标出任何渐近线的方程、与坐标轴交点的坐标以及转折点的坐标(若适用)。

  1. (a)

    y=f(x)y = f(x)

  2. (b)

    y=1f′(x)y = \dfrac{1}{f'(x)}

  3. (c)

    y=−f(∣x−1∣)y = -f(|x-1|)

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完整解法

1(a)

1. Deduce the graph of $y = f(x)$ for $x \ge 0$ from $y = f(|x|)$.

For x≥0x \ge 0, the transformation y=f(∣x∣)y = f(|x|) simplifies to y=f(x)y = f(x). Thus, the right portion of the graph of y=f(x)y = f(x) is identical to the right portion of the given y=f(∣x∣)y = f(|x|) graph.
From the given information, we deduce the following features for x≥0x \ge 0:

  • Vertical asymptote: x=1x = 1
  • Horizontal asymptote: y=2y = 2 (as x→∞x \to \infty)
  • Axial intercept: (2,0)(2, 0)
  • Turning point: Local maximum at (3,4)(3, 4)

To connect the intercept at x=2x=2 up to the peak at x=3x=3, the graph must approach −∞-\infty as x→1+x \to 1^+.

2. Deduce the graph of $y = f(x)$ for $x < 1$ using $y = f'(x)$.

The vertical asymptote x=−1x = -1 on f(∣x∣)f(|x|) is merely a reflection; f(x)f(x) itself only has a vertical asymptote at x=1x = 1.
From the description of y=f′(x)y = f'(x), its only xx-intercept is at x=3x = 3. This means f(x)f(x) has no turning points for x<1x < 1.
Since f′(x)f'(x) has an asymptote at y=0y = 0, f(x)f(x) approaches a horizontal asymptote as x→−∞x \to -\infty, which must be y=2y = 2 (consistent with f(∣x∣)f(|x|)).
Because f(x)f(x) approaches y=2y=2 and has no turning points for x<1x<1, it is strictly monotonic. Given that typical rational functions with a single vertical asymptote and a horizontal asymptote have an even-multiplicity pole (to go to the same infinity on both sides and avoid slant asymptotes), f(x)→−∞f(x) \to -\infty as x→1−x \to 1^-.
Consequently, f(x)f(x) monotonically decreases from y=2y=2 down to −∞-\infty, crossing the negative xx-axis and negative yy-axis.

3. Sketch the complete graph of $y = f(x)$.

Combine the deduced features to sketch the full graph of y=f(x)y = f(x):

  1. Draw and label the asymptotes x=1x = 1 and y=2y = 2 with dashed lines.
  2. Plot and label the known coordinates: the xx-intercept at (2,0)(2,0) and the turning point at (3,4)(3,4).
  3. Sketch the right branch (x>1x > 1) coming from −∞-\infty at x=1+x=1^+, passing through (2,0)(2,0), turning smoothly at (3,4)(3,4), and approaching y=2y=2 from above.
  4. Sketch the left branch (x<1x < 1) as a strictly decreasing curve starting from just below y=2y=2 at the far left, crossing the negative axes, and going down to −∞-\infty at x=1−x=1^-.
答案

Sketch of y=f(x)y = f(x):

  • Axes & Asymptotes: Draw xx- and yy-axes. Draw dashed lines for the vertical asymptote x=1x = 1 and the horizontal asymptote y=2y = 2, labelling them with their equations.
  • For x>1x > 1: The curve comes up from x=1x = 1 (near −∞-\infty), crosses the xx-axis at (2,0)(2, 0), reaches a local maximum at (3,4)(3, 4), and then curves downwards to approach y=2y = 2 from above as x→∞x \to \infty.
  • For x<1x < 1: The curve starts just below y=2y = 2 on the far left (x→−∞x \to -\infty), strictly decreases to cross the negative xx-axis and negative yy-axis, and approaches −∞-\infty as x→1−x \to 1^-.
    (Clearly label the asymptote equations x=1x=1, y=2y=2 and the key coordinates (2,0)(2,0) and (3,4)(3,4).)

1(b)

1. Identify the axial intercepts of the reciprocal graph.

The graph of f′(x)f'(x) has a vertical asymptote at x=1x=1, meaning ∣f′(x)∣→∞|f'(x)| \to \infty as x→1x \to 1. Consequently, its reciprocal 1f′(x)→0\frac{1}{f'(x)} \to 0.
This provides an x-intercept for the reciprocal graph at the point (1,0)(1, 0).

2. Identify the vertical and horizontal asymptotes.

The graph of f′(x)f'(x) crosses the x-axis at x=3x=3, so f′(3)=0f'(3) = 0. The reciprocal 1f′(x)\frac{1}{f'(x)} will therefore have a vertical asymptote at x=3x=3.
Furthermore, since f′(x)f'(x) has a horizontal asymptote of y=0y=0 (meaning f′(x)→0f'(x) \to 0 as x→±∞x \to \pm\infty), the reciprocal 1f′(x)→±∞\frac{1}{f'(x)} \to \pm\infty as x→±∞x \to \pm\infty. Thus, the reciprocal graph has no horizontal asymptotes.

3. Determine the shape of the curve and any unmarked turning points.

  • For 1<x<31 < x < 3: f′(x)f'(x) is positive and decreases from +∞+\infty to 00. Thus, 1f′(x)\frac{1}{f'(x)} increases from 00 (at x=1x=1) to +∞+\infty (as x→3−x \to 3^-).
  • For x>3x > 3: f′(x)f'(x) crosses into negative values but must approach 00 as x→∞x \to \infty. By Rolle's Theorem/logic, it must reach a negative local minimum before turning back towards y=0y=0. Consequently, 1f′(x)\frac{1}{f'(x)} starts from −∞-\infty at x=3+x=3^+, goes up to reach a negative local maximum, and then decreases back to −∞-\infty as x→∞x \to \infty.
  • For x<1x < 1: f′(x)f'(x) does not cross the x-axis and approaches 00 as x→−∞x \to -\infty. Thus 1f′(x)\frac{1}{f'(x)} connects from (1,0)(1,0) and monotonically tends towards ±∞\pm\infty as x→−∞x \to -\infty.
答案

A valid sketch must include:

  • x-intercept at (1,0)(1, 0).
  • Vertical asymptote at x=3x = 3.
  • No horizontal asymptotes.
  • Curve shape:
    • For x∈(1,3)x \in (1, 3), the curve starts from (1,0)(1,0) and goes to +∞+\infty as x→3−x \to 3^-.
    • For x>3x > 3, the curve comes from −∞-\infty at x=3+x=3^+, reaches a negative local maximum (coordinates not required), and then goes down to −∞-\infty as x→∞x \to \infty.
    • For x<1x < 1, the curve emerges from (1,0)(1,0) and goes to ±∞\pm\infty as x→−∞x \to -\infty (following the original visual sign of f′(x)f'(x)).

1(c)

1. Determine the sequence of transformations required.

Let g(x)=f(∣x∣)g(x) = f(|x|). The target function is y=−f(∣x−1∣)=−g(x−1)y = -f(|x-1|) = -g(x-1).
This requires two transformations applied to the graph of g(x)g(x):

  1. Replace xx with x−1x-1: a horizontal translation of 11 unit in the positive xx-direction.
  2. Multiply the function by −1-1: a reflection about the xx-axis.

2. Apply the transformations to the turning points and $x$-intercepts.

Turning points:
Start: (−3,4)(-3, 4) and (3,4)(3, 4)
Translate right by 1: (−2,4)(-2, 4) and (4,4)(4, 4)
Reflect in xx-axis: (−2,−4)(-2, -4) and (4,−4)(4, -4)

xx-axis intercepts:
Start: (−2,0)(-2, 0) and (2,0)(2, 0)
Translate right by 1: (−1,0)(-1, 0) and (3,0)(3, 0)
Reflect in xx-axis: The yy-coordinates are 00, so they remain at (−1,0)(-1, 0) and (3,0)(3, 0).

3. Apply the transformations to the asymptotes.

Vertical Asymptotes:
Start: x=−1x = -1 and x=1x = 1
Translate right by 1: x=0x = 0 and x=2x = 2
Reflect in xx-axis: Vertical lines are unchanged, remaining x=0x = 0 and x=2x = 2.

Horizontal Asymptote:
Start: y=2y = 2
Translate right by 1: Unchanged, y=2y = 2
Reflect in xx-axis: y=−2y = -2

答案

Sketch a graph with the following labelled features:

  • Turning points (minima): (−2,−4)(-2, -4) and (4,−4)(4, -4)
  • xx-axis intercepts: (−1,0)(-1, 0) and (3,0)(3, 0)
  • Vertical asymptotes: x=0x = 0 (the yy-axis) and x=2x = 2
  • Horizontal asymptote: y=−2y = -2

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