[在此问题中,所给图像的草图未按比例绘制] y=∣f(x)∣y = |\mathrm{f}(x)| 和 y2=f(x)y^2 = \mathrm{f}(x) 的图像如下所示。 在不同的图上,分别画出下列函数的图像草图, 写出所有渐近线方程以及与坐标轴交点的坐标。

  1. (a)

    y2=f(−2x)y^2 = \mathrm{f}(-2x),

  2. (b)

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

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完整解法

1(a)

1. Identify the transformation from $y^2 = \mathrm{f}(x)$ to $y^2 = \mathrm{f}(-2x)$.

The equation y2=f(−2x)y^2 = \mathrm{f}(-2x) replaces xx with −2x-2x. This corresponds to a horizontal scaling by a factor of 12\frac{1}{2} and a reflection in the yy-axis.

2. Determine the new asymptote and $x$-intercept.

Applying the transformation to the key features of y2=f(x)y^2 = \mathrm{f}(x):

  • Vertical asymptote: x=3  ⟹  −2x=3  ⟹  x=−32x = 3 \implies -2x = 3 \implies x = -\frac{3}{2}
  • xx-intercept (vertex): x=5  ⟹  −2x=5  ⟹  x=−52x = 5 \implies -2x = 5 \implies x = -\frac{5}{2}, giving the coordinate (−52,0)(-\frac{5}{2}, 0)

3. Determine the shape and domain of the new graph.

The original graph exists for 3<x≤53 < x \le 5 and opens to the left from (5,0)(5, 0).
For the transformed graph, the domain is 3<−2x≤5  ⟹  −52≤x<−323 < -2x \le 5 \implies -\frac{5}{2} \le x < -\frac{3}{2}.
The new graph consists of two branches starting at (−52,0)(-\frac{5}{2}, 0) and opening to the right towards the vertical asymptote x=−32x = -\frac{3}{2}, maintaining symmetry about the xx-axis.

答案

Sketch of y2=f(−2x)y^2 = \mathrm{f}(-2x):

  • A curve symmetric about the xx-axis consisting of two branches.
  • The vertex (xx-intercept) is at (−52,0)(-\frac{5}{2}, 0).
  • The curve opens to the right from the vertex.
  • There is a vertical asymptote at x=−32x = -\frac{3}{2}, which the branches approach as y→±∞y \to \pm\infty.
  • No yy-intercepts.

1(b)

1. Determine the intervals where $\mathrm{f}(x)$ is positive or negative from the $y^2 = \mathrm{f}(x)$ graph.

The relation y2=f(x)y^2 = \mathrm{f}(x) only yields real values of yy when f(x)≥0\mathrm{f}(x) \ge 0.
Given that the graph of y2=f(x)y^2 = \mathrm{f}(x) consists of branches starting at (5,0)(5,0) and opening to the left towards the asymptote x=3x=3, this curve only exists for 3<x≤53 < x \le 5.
Furthermore, it is explicitly defined "only for x>3x > 3", ruling out any regions x<3x < 3.
Therefore, we deduce:

  • f(x)≥0\mathrm{f}(x) \ge 0 for 3<x≤53 < x \le 5
  • f(x)≤0\mathrm{f}(x) \le 0 for x<3x < 3 and x>5x > 5

2. Relate $y = \mathrm{f}(x)$ to the provided graph of $y = |\mathrm{f}(x)|$.

Using the signs of f(x)\mathrm{f}(x), we construct y=f(x)y = \mathrm{f}(x) from y=∣f(x)∣y = |\mathrm{f}(x)|:

  • For 3≤x≤53 \le x \le 5: Since f(x)≥0\mathrm{f}(x) \ge 0, f(x)=∣f(x)∣\mathrm{f}(x) = |\mathrm{f}(x)|. The sketch of y=f(x)y = \mathrm{f}(x) in this region is identical to the graph of y=∣f(x)∣y = |\mathrm{f}(x)|.
  • For x<3x < 3 and x>5x > 5: Since f(x)≤0\mathrm{f}(x) \le 0, f(x)=−∣f(x)∣\mathrm{f}(x) = -|\mathrm{f}(x)|. The sketch is obtained by reflecting the corresponding parts of the graph of y=∣f(x)∣y = |\mathrm{f}(x)| in the xx-axis.

Note that the sharp cusp on y=∣f(x)∣y = |\mathrm{f}(x)| at x=5x = 5 unfolds into a smooth curve crossing the xx-axis in y=f(x)y = \mathrm{f}(x), which matches f(x)\mathrm{f}(x) changing sign from positive to negative.

3. Determine the exact asymptotes and intercepts.

Applying the reflections in the xx-axis for x<3x < 3 and x>5x > 5 to the key features of the ∣f(x)∣|\mathrm{f}(x)| graph:

  • The horizontal asymptote y=1y = 1 reflects to become y=−1y = -1.
  • The yy-intercept (0,2)(0, 2) reflects to become (0,−2)(0, -2).
  • The known xx-intercept (5,0)(5, 0) remains (5,0)(5, 0).
  • The smooth local minimum touching the xx-axis at the negative root remains an xx-intercept, becoming a local maximum.
  • The vertical asymptotes x=1x = 1 and x=3x = 3 are unchanged.
答案

Sketch of y=f(x)y = \mathrm{f}(x):
[A valid sketch should show the following features on a coordinate plane]

  • x<1x < 1: Curve approaching y=−1y = -1 from above as x→−∞x \to -\infty, rising to touch the xx-axis at a negative xx-value (smooth local maximum), falling to cross the yy-axis at (0,−2)(0, -2), and tending to −∞-\infty as x→1−x \to 1^-.
  • 1<x<31 < x < 3: An inverted U-shaped curve entirely below the xx-axis, tending to −∞-\infty as x→1+x \to 1^+ and as x→3−x \to 3^-.
  • x>3x > 3: Curve starting from +∞+\infty near x=3+x = 3^+, falling to cross the xx-axis smoothly at (5,0)(5, 0), and continuing to decrease while approaching y=−1y = -1 from above as x→∞x \to \infty.

Equations of Asymptotes:
x=1x = 1, x=3x = 3, y=−1y = -1

Coordinates of Points of Intersection:
yy-axis: (0,−2)(0, -2)
xx-axis: (5,0)(5, 0) and an unknown negative coordinate (x0,0)(x_0, 0).

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