下图显示了 y = f(x) 的图像。曲线在 (-2, 0) 和 (1, 0) 处穿过 x 轴,并且具有坐标为 (2, 2) 的极大值点。直线 x = -1 和 y = 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,画出以下函数的图像:

  1. (a)

    y = 3 f(x + a),其中 a 是一个正常数且满足 1 < a < 2 ,

  2. (b)

    y = f'(x)。

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完整解法

2(a)

1. Identify the geometrical transformations represented by the equation.

The equation is y=3f(x+a)y = 3\mathrm{f}(x + a). Compared to the original graph of y=f(x)y = \mathrm{f}(x), this represents:

  1. A horizontal translation by aa units in the negative xx-direction (to the left).
  2. A vertical stretch by a scale factor of 3 parallel to the yy-axis.

2. Determine the new equations of the asymptotes.

Applying the transformations to the original asymptotes:

  • The vertical asymptote x=−1x = -1 is translated aa units left, becoming x=−1−ax = -1 - a.
  • The horizontal asymptote y=1y = 1 is unaffected by the horizontal translation but is stretched vertically by a factor of 3, becoming y=3(1)=3y = 3(1) = 3.

3. Find the coordinates of the transformed $x$-intercepts and stationary point, and verify their graphical positions.

Applying the transformations to the known points:

  • For the xx-intercepts (−2,0)(-2, 0) and (1,0)(1, 0), the yy-coordinates remain 00. The xx-coordinates are translated to give new intercepts at (−2−a,0)(-2 - a, 0) and (1−a,0)(1 - a, 0).
  • For the maximum point (2,2)(2, 2), the xx-coordinate is shifted by aa and the yy-coordinate is multiplied by 33. The new maximum is at (2−a,6)(2 - a, 6).

Given the condition 1<a<21 < a < 2:

  • 1−a1 - a is between −1-1 and 00, so the intercept (1−a,0)(1 - a, 0) must be drawn on the negative xx-axis.
  • 2−a2 - a is between 00 and 11, so the maximum point (2−a,6)(2 - a, 6) must be drawn in the first quadrant (to the right of the yy-axis).
  • Since the maximum is to the right of the yy-axis and the xx-intercept is to the left, the curve's yy-intercept will be positive.
答案

A sketch of the curve with the following features:

  • Asymptotes: Vertical line x=−1−ax = -1 - a and horizontal line y=3y = 3.
  • xx-intercepts: (−2−a,0)(-2 - a, 0) and (1−a,0)(1 - a, 0). Note that since 1<a<21 < a < 2, the intercept (1−a,0)(1 - a, 0) is on the negative xx-axis.
  • Maximum point: (2−a,6)(2 - a, 6). Since 1<a<21 < a < 2, this point lies in the first quadrant.
  • yy-intercept: The curve intersects the positive yy-axis.
  • The shape is vertically stretched and shifted left, preserving the two-branch structure of the original curve.

2(b)

1. Determine the coordinates of the intersection with the $x$-axis.

The graph of y=f(x)y = f(x) has a maximum point at (2,2)(2, 2), which means the gradient at x=2x = 2 is zero.
Therefore, f′(2)=0f'(2) = 0, giving an xx-intercept at (2,0)(2, 0) for the graph of y=f′(x)y = f'(x).

2. Determine the equations of the asymptotes.

As x→±∞x \to \pm\infty, the graph of f(x)f(x) approaches the horizontal asymptote y=1y = 1, meaning the gradient approaches 00. Thus, y=0y = 0 (the xx-axis) is a horizontal asymptote for f′(x)f'(x).
As x→−1x \to -1, f(x)f(x) approaches the vertical asymptote x=−1x = -1. Because the curve falls to −∞-\infty on both sides of x=−1x = -1, the gradient f′(x)→−∞f'(x) \to -\infty as x→−1−x \to -1^- and f′(x)→+∞f'(x) \to +\infty as x→−1+x \to -1^+. Thus, x=−1x = -1 is a vertical asymptote for f′(x)f'(x).

3. Determine the signs of $f'(x)$ and the presence of stationary points to deduce the final shape.

By observing the increasing/decreasing nature of f(x)f(x):

  • For x<−1x < -1, f(x)f(x) is decreasing   ⟹  f′(x)<0\implies f'(x) < 0 (curve below xx-axis).
  • For −1<x<2-1 < x < 2, f(x)f(x) is increasing   ⟹  f′(x)>0\implies f'(x) > 0 (curve above xx-axis).
  • For x>2x > 2, f(x)f(x) is decreasing   ⟹  f′(x)<0\implies f'(x) < 0 (curve below xx-axis).

Since f′(2)=0f'(2) = 0 and f′(x)→0f'(x) \to 0 as x→∞x \to \infty, the continuous graph of f′(x)f'(x) must reach a local minimum at some point where x>2x > 2 before approaching the xx-axis from below. (Exact coordinates of this stationary point and the yy-intercept cannot be deduced).

答案

Features to be shown on the sketch of y=f′(x)y = f'(x):

  • xx-intercept: (2,0)(2, 0)
  • Vertical asymptote: x=−1x = -1
  • Horizontal asymptote: y=0y = 0
  • Turning point: A local minimum for some x>2x > 2 (exact coordinates cannot be determined).
  • Shape and Signs:
    • x<−1x < -1: Curve is entirely below the xx-axis, starting near y=0y = 0 as x→−∞x \to -\infty and going to −∞-\infty as x→−1−x \to -1^-.
    • −1<x<2-1 < x < 2: Curve is entirely above the xx-axis, coming from +∞+\infty as x→−1+x \to -1^+ and decreasing to intersect the xx-axis at x=2x = 2.
    • x>2x > 2: Curve is below the xx-axis, decreasing to a minimum before increasing back towards y=0y = 0 as x→∞x \to \infty.

(A sketch incorporating these features would be drawn on the Cartesian plane.)

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