图示为曲线 y=f(1−x)y = \text{f}(1-x)。该曲线的极大值点为 (−1,5)(-1, 5),且有两条渐近线 x=−2x = -2 and y=1y = 1。 在不同的图上,分别画出下列函数的图像草图:

  1. (a)

    y=f(x)y = \text{f}(x),

  2. (b)

    y=g′(x)y = \text{g}'(x),其中 g(x)=f(1−x)\text{g}(x) = \text{f}(1-x)。

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完整解法

3(a)

1. Establish the algebraic mapping between the given graph and the required graph.

We are given the graph of y=f(1−x)y = \text{f}(1-x) and need to sketch y=f(x)y = \text{f}(x).
Let a point on the given graph be (x,y)(x, y). This means y=f(1−x)y = \text{f}(1-x).
Let X=1−xX = 1 - x. Then the equation becomes y=f(X)y = \text{f}(X).
Therefore, a point (x,y)(x, y) on the given curve maps to the point (1−x,y)(1-x, y) on the target curve y=f(x)y = \text{f}(x).

2. Apply the mapping to the key features of the graph (turning point and asymptotes).

Apply the mapping X=1−xX = 1 - x to the given key features:

  • Maximum point: For x=−1,y=5  ⟹  X=1−(−1)=2x = -1, y = 5 \implies X = 1 - (-1) = 2. The new maximum point is (2,5)(2, 5).
  • Vertical asymptote: For x=−2  ⟹  X=1−(−2)=3x = -2 \implies X = 1 - (-2) = 3. The new vertical asymptote is x=3x = 3.
  • Horizontal asymptote: The yy-values are unaffected by horizontal transformations, so the horizontal asymptote remains y=1y = 1.

3. Determine the orientation and shape of the transformed branches.

The transformation x↦1−xx \mapsto 1-x effectively reverses the horizontal direction of the graph (a reflection in the vertical line x=0.5x = 0.5).

  • The original right branch (for x>−2x > -2) now becomes the new left branch (for X<3X < 3). As x→∞x \to \infty, y→1+y \to 1^+, so as X→−∞X \to -\infty, y→1+y \to 1^+. The branch rises to the maximum at (2,5)(2, 5) and plunges to −∞-\infty as X→3−X \to 3^-.
  • The original left branch (for x<−2x < -2) now becomes the new right branch (for X>3X > 3). As x→−∞x \to -\infty, y→1+y \to 1^+, so as X→∞X \to \infty, y→1+y \to 1^+. The branch descends from +∞+\infty near X=3+X = 3^+ and flattens out towards y=1y = 1.
答案

Graph Sketch Instructions:

  • Axes & Asymptotes: Draw a horizontal dashed line for the asymptote y=1y = 1 and a vertical dashed line for the asymptote x=3x = 3.
  • Turning Point: Plot and label the local maximum point at (2,5)(2, 5).
  • Curve Shape:
    • Left Branch (x<3x < 3): Draw a curve starting near y=1y = 1 on the far left, rising to the maximum point at (2,5)(2, 5), then curving steeply downwards to negative infinity as it approaches x=3x = 3 from the left.
    • Right Branch (x>3x > 3): Draw a curve starting from positive infinity just to the right of x=3x = 3, curving downwards to asymptotically approach y=1y = 1 as x→+∞x \to +\infty.

3(b)

1. Recognize the relationship between $\text{g}(x)$ and the given graph.

The question defines g(x)=f(1−x)\text{g}(x) = \text{f}(1-x). Since the given diagram is exactly the graph of y=f(1−x)y = \text{f}(1-x), the given graph is y=g(x)y = \text{g}(x). We must sketch the derivative of this exact graph.

2. Identify the $x$-intercepts of $y = \text{g}'(x)$.

The graph of y=g(x)y = \text{g}(x) has a maximum turning point at (−1,5)(-1, 5). At this point, the gradient is zero, which means g′(−1)=0\text{g}'(-1) = 0. Therefore, y=g′(x)y = \text{g}'(x) has an xx-intercept at (−1,0)(-1, 0).

3. Determine the asymptotes of $y = \text{g}'(x)$.

As x→−2x \to -2 from both the left and the right, the curve of y=g(x)y = \text{g}(x) becomes increasingly steep with a positive slope, meaning the gradient approaches +∞+\infty. Thus, x=−2x = -2 is a vertical asymptote for y=g′(x)y = \text{g}'(x), with the curve tending to +∞+\infty on both sides.

As x→±∞x \to \pm\infty, the graph of y=g(x)y = \text{g}(x) flattens out as it approaches the horizontal asymptote y=1y = 1. This means the gradient approaches 00. Thus, y=0y = 0 (the xx-axis) is a horizontal asymptote for y=g′(x)y = \text{g}'(x).

4. Determine the intervals of positive and negative gradient to finalize the shape.

For x<−2x < -2 and −2<x<−1-2 < x < -1, the function y=g(x)y = \text{g}(x) is strictly increasing, so g′(x)>0\text{g}'(x) > 0 (the graph lies above the xx-axis).
For x>−1x > -1, y=g(x)y = \text{g}(x) is strictly decreasing, so g′(x)<0\text{g}'(x) < 0 (the graph lies below the xx-axis).
Because g′(x)\text{g}'(x) must return to 00 as x→∞x \to \infty, the branch for x>−1x > -1 must have a local minimum point below the xx-axis before approaching y=0y = 0.

答案

A sketch of y=g′(x)y = \text{g}'(x) showing:

  • A vertical asymptote at x=−2x = -2 with both adjacent branches tending to +∞+\infty.
  • A horizontal asymptote at y=0y = 0 (the xx-axis).
  • An xx-intercept at (−1,0)(-1, 0).
  • For x<−2x < -2, the curve lies above the xx-axis, increasing from y=0y=0 towards +∞+\infty.
  • For x>−2x > -2, the curve descends from +∞+\infty, passes through (−1,0)(-1, 0) to a local minimum below the xx-axis, and then approaches y=0y=0 from below as x→∞x \to \infty.

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