下图显示了 y=f(x)y = \text{f}(x) 的图像。该曲线穿过 xx 轴的点为 (−2,0)(-2, 0) 和 (1,0)(1, 0),并且有一个最大值点,其坐标为 (2,2)(2, 2)。直线 x=−1x = -1 和 y=1y = 1 是该图像的渐近线。 在可能的情况下,指出所有渐近线的方程、与坐标轴的交点坐标以及驻点坐标,并画出以下函数的图像:

  1. (a)

    y=3f(x+a)y = 3\text{f}(x + a),其中 aa 是一个满足 1<a<21 < a < 2 的正常数,

  2. (b)

    y=f′(x)y = \text{f}'(x)。

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完整解法

2(a)

1. Identify the geometrical transformations required

The equation y=3f(x+a)y = 3\text{f}(x + a) indicates two transformations on the graph of y=f(x)y = \text{f}(x):

  1. A translation by aa units in the negative xx-direction (shift left by aa).
  2. A vertical stretch by a scale factor of 33 parallel to the yy-axis.

2. Find the equations of the new asymptotes

Original vertical asymptote: x=−1x = -1
Translated left by aa:
x=−1−ax = -1 - a

Original horizontal asymptote: y=1y = 1
Stretched vertically by 3:
y=1×3  ⟹  y=3y = 1 \times 3 \implies y = 3

3. Find the coordinates of the new stationary point

Original maximum point: (2,2)(2, 2)
Translated left by aa and stretched vertically by 3:
(x,y)→(2−a,2×3)  ⟹  (2−a,6)(x, y) \to (2 - a, 2 \times 3) \implies (2 - a, 6)
Since 1<a<21 < a < 2, we have 0<2−a<10 < 2 - a < 1, so the new maximum point lies in the first quadrant.

4. Determine the new intersections with the axes

Original xx-intercepts: (−2,0)(-2, 0) and (1,0)(1, 0)
Translated left by aa (vertical stretch does not affect xx-intercepts):
(−2−a,0)and(1−a,0)(-2 - a, 0) \quad \text{and} \quad (1 - a, 0)
Since 1<a<21 < a < 2, we have −1<1−a<0-1 < 1 - a < 0, meaning both xx-intercepts lie on the negative xx-axis.

The new yy-intercept occurs at x=0x = 0, giving y=3f(a)y = 3\text{f}(a). From the original graph, for 1<x<21 < x < 2, f(x)>0\text{f}(x) > 0. Hence f(a)>0\text{f}(a) > 0, and the curve intersects the positive yy-axis (the exact coordinate cannot be determined).

5. Sketch the final transformed graph

Sketch the two branches based on the transformed features:

  • Left branch: Bounded by x=−1−ax = -1 - a and y=3y = 3, passing through (−2−a,0)(-2-a, 0).
  • Right branch: Starts near the bottom of x=−1−ax = -1 - a, crosses the xx-axis at (1−a,0)(1-a, 0), crosses the positive yy-axis, peaks at the maximum (2−a,6)(2-a, 6), and approaches y=3y = 3 from above as x→∞x \to \infty.
答案

Sketch Features:

  • Shape: Two branches similar to y=f(x)y = \text{f}(x).
  • Asymptotes: Vertical line x=−1−ax = -1 - a and horizontal line y=3y = 3.
  • Axes Intersections: xx-intercepts at (−2−a,0)(-2-a, 0) and (1−a,0)(1-a, 0). The graph intersects the positive yy-axis.
  • Stationary Point: Maximum point at (2−a,6)(2-a, 6).

2(b)

1. Determine the equations of the asymptotes for $y = \text{f}'(x)$.

Since the graph of y=f(x)y = \text{f}(x) has a vertical asymptote at x=−1x = -1, the gradient is undefined and its magnitude approaches infinity. Thus, x=−1x = -1 is a vertical asymptote for y=f′(x)y = \text{f}'(x).
As x→±∞x \to \pm \infty, the curve approaches the horizontal asymptote y=1y = 1, meaning the curve flattens out and the gradient approaches 00. Thus, y=0y = 0 (the xx-axis) is a horizontal asymptote for y=f′(x)y = \text{f}'(x).

2. Determine the coordinates of intersection with the axes.

At the maximum point (2,2)(2, 2) on y=f(x)y = \text{f}(x), the gradient is 00. Therefore, f′(2)=0\text{f}'(2) = 0, giving an xx-intercept at (2,0)(2, 0).
The exact yy-intercept cannot be calculated from the given information, but it must be positive since f(x)\text{f}(x) is strictly increasing at x=0x = 0.

3. Analyse the sign of the gradient in each interval to deduce the shape of the derivative graph.

For x<−1x < -1, f(x)\text{f}(x) is strictly decreasing, so f′(x)<0\text{f}'(x) < 0. The curve comes from y=0y = 0 as x→−∞x \to -\infty and plunges to −∞-\infty as x→−1−x \to -1^-.
For −1<x<2-1 < x < 2, f(x)\text{f}(x) is strictly increasing, so f′(x)>0\text{f}'(x) > 0. The curve falls from ∞\infty as x→−1+x \to -1^+, and decreases to cross the xx-axis at (2,0)(2, 0).
For x>2x > 2, f(x)\text{f}(x) is strictly decreasing, so f′(x)<0\text{f}'(x) < 0. The curve dips below the xx-axis. Since it must approach the horizontal asymptote y=0y = 0 as x→∞x \to \infty, it must reach a local minimum point before turning upwards. (The exact coordinates of this minimum cannot be determined).

答案

Asymptotes: x=−1x = -1, y=0y = 0
Points of intersection: (2,0)(2, 0) on the xx-axis. (The exact yy-intercept cannot be determined, but must be plotted as positive).
Stationary points: A local minimum exists for some x>2x > 2 (exact coordinates cannot be determined).

Sketch instructions:

  • Left branch (x<−1x < -1): Drawn entirely below the xx-axis. Approaches y=0y = 0 from below as x→−∞x \to -\infty, and approaches x=−1x = -1 going towards −∞-\infty.
  • Right branch (x>−1x > -1): Comes down from ∞\infty near x=−1+x = -1^+, crosses the yy-axis at a positive value, crosses the xx-axis at (2,0)(2, 0), reaches a local minimum (with a negative yy-value), and curves back up to approach y=0y = 0 from below as x→∞x \to \infty.

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