图中显示了曲线 y=f(x)y = f(x),其极值点为 A(3,2.5)A(3, 2.5) 和 B(0,1)B(0, 1)。直线 y=1y = 1 和 x=−2x = -2 是该曲线的渐近线。

  1. (a)

    在不同的图上分别画出下列函数的图像,并视情况清晰写出渐近线的方程以及对应于 A 和 B 的点的坐标。

    1. (i)

      y=f(x−2)+2y = f(x-2)+2

    2. (ii)

      y=f′(x)y = f'(x)

  2. (b)

    已知一曲线的参数方程为

    x=t2−2t,y=t3−3t,for t>0x = t^2 - 2t, \qquad y = t^3 - 3t, \qquad \text{for } t > 0。

    该曲线关于 yy 轴反射。求反射后的曲线与 xx 轴交点的精确坐标。

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完整解法

4(a)(i)

1. Identify the geometrical transformations applied to the original graph.

The equation y=f(x−2)+2y = f(x-2) + 2 represents a translation of the graph of y=f(x)y = f(x) by 22 units in the positive xx-direction (to the right) and 22 units in the positive yy-direction (upwards).

2. Find the coordinates of the new turning points.

Point A(3,2.5)→A′(3+2,2.5+2)=(5,4.5)A(3, 2.5) \rightarrow A'(3+2, 2.5+2) = (5, 4.5)
Point B(0,1)→B′(0+2,1+2)=(2,3)B(0, 1) \rightarrow B'(0+2, 1+2) = (2, 3)

3. Find the equations of the new asymptotes.

Vertical asymptote: x=−2→x=−2+2  ⟹  x=0x = -2 \rightarrow x = -2 + 2 \implies x = 0 (the yy-axis)
Horizontal asymptote: y=1→y=1+2  ⟹  y=3y = 1 \rightarrow y = 1 + 2 \implies y = 3

答案

A sketch of the curve with the same shape as y=f(x)y = f(x), but shifted. The new turning points are (5,4.5)(5, 4.5) and (2,3)(2, 3). The new asymptotes are x=0x = 0 (the yy-axis) and y=3y = 3.

4(a)(ii)

1. Determine the asymptotes of y = f'(x)

The vertical asymptote of ff is unchanged, so x=−2x=-2 is still a vertical asymptote of f′(x)f'(x); near it ff increases steeply to +∞+\infty, so f′(x)→−∞f'(x)\to-\infty as x→−2+x\to-2^+.

Since y=1y=1 is a horizontal asymptote of ff, the curve flattens as x→+∞x\to+\infty, so f′(x)→0f'(x)\to0. Hence the horizontal asymptote of f′(x)f'(x) is y=0y=0 (the xx-axis).

2. Convert turning points of f into x-intercepts of f'

At a turning point the gradient is zero, so f′(x)=0f'(x)=0 at the xx-coordinates of AA and BB.

  • B(0,1)⇒B(0,1)\Rightarrow xx-intercept (0,0)(0,0).
  • A(3,2.5)⇒A(3,2.5)\Rightarrow xx-intercept (3,0)(3,0).

3. Use sign of the gradient on each interval to sketch f'

Read off the gradient of ff on each interval (B is a minimum, A is a maximum):

  • −2<x<0-2<x<0: ff decreasing ⇒f′(x)<0\Rightarrow f'(x)<0 (below axis, from −∞-\infty up to 00 at x=0x=0).
  • 0<x<30<x<3: ff increasing ⇒f′(x)>0\Rightarrow f'(x)>0 (above axis, a positive hump).
  • x>3x>3: ff decreasing towards y=1y=1 ⇒f′(x)<0\Rightarrow f'(x)<0, approaching 0−0^-.

Sketch a curve rising from −∞-\infty near x=−2x=-2, crossing the axis at (0,0)(0,0), peaking, crossing back at (3,0)(3,0), then staying just below the xx-axis and tending to y=0y=0.

答案

Sketch of y=f′(x)y=f'(x) with:

  • Vertical asymptote x=−2x=-2 (curve →−∞\to-\infty as x→−2+x\to-2^+).
  • Horizontal asymptote y=0y=0 (curve →0−\to0^- as x→+∞x\to+\infty).
  • xx-intercepts at (0,0)(0,0) (from BB) and (3,0)(3,0) (from AA).
  • f′(x)<0f'(x)<0 for −2<x<0-2<x<0, f′(x)>0f'(x)>0 for 0<x<30<x<3, f′(x)<0f'(x)<0 for x>3x>3.

4(b)

1. Set $y = 0$ to find the value of the parameter $t$ at the $x$-axis intercept.

y=t3−3t=0y = t^3 - 3t = 0
t(t2−3)=0t(t^2 - 3) = 0
Since the domain restricts t>0t > 0, we have t2=3  ⟹  t=3t^2 = 3 \implies t = \sqrt{3}.

2. Find the original $x$-coordinate, apply the reflection in the $y$-axis, and state the final exact coordinates.

Original xx-coordinate at t=3t = \sqrt{3}:
x=(3)2−2(3)=3−23x = (\sqrt{3})^2 - 2(\sqrt{3}) = 3 - 2\sqrt{3}
When a curve is reflected in the yy-axis, its yy-coordinates remain unchanged and its xx-coordinates are negated.
Reflected xx-coordinate:
−x=−(3−23)=23−3-x = -(3 - 2\sqrt{3}) = 2\sqrt{3} - 3
The exact coordinates of the point are (23−3,0)(2\sqrt{3} - 3, 0).

答案

(23−3,0)(2\sqrt{3} - 3, 0)

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