曲线 C1C_1 的方程为 p2x2−y2=p2p^2x^2 - y^2 = p^2,其中 p>1p > 1。

  1. ((i))

    画出 C1C_1 的草图,用 pp 表示出其与坐标轴的所有交点坐标、所有驻点坐标以及所有渐近线方程。

  2. ((ii))

    C1C_1 经过单一变换得到 C2C_2。已知 C2C_2 有一条对称轴 x=2x = 2,且点 (4,3)(4,3) 在 C2C_2 上,求 pp 的值。

  3. ((iii))

    下面给出了 y=f(x)y = f(x) 的图像。它在 x=−1x = -1 处有一个极大值点,且有一条水平渐近线 y=0y = 0。

    在包含 C1C_1 的同一坐标系中,画出 y=f′(x)y = f'(x) 的图像草图。

    由此,说明方程 p2x2−[f′(x)]2=p2 p^2x^2 - [f'(x)]^2 = p^2\, 的根的个数。

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完整解法

7((i))

1. Rewrite the equation into the standard form of a hyperbola.

p2x2−y2=p2p^2x^2 - y^2 = p^2
Dividing through by p2p^2:
x2−y2p2=1x^2 - \frac{y^2}{p^2} = 1
This represents a standard hyperbola with a horizontal transverse axis.

2. Find the coordinates of the points of intersection with the axes.

For xx-intercepts, set y=0y = 0:
p2x2=p2  ⟹  x2=1  ⟹  x=±1p^2x^2 = p^2 \implies x^2 = 1 \implies x = \pm 1
The intersections with the xx-axis are (1,0)(1, 0) and (−1,0)(-1, 0).

For yy-intercepts, set x=0x = 0:
−y2=p2-y^2 = p^2
Since p>1p > 1, p2>0p^2 > 0, giving y2<0y^2 < 0 which has no real solutions. There are no intersections with the yy-axis.

3. Determine the coordinates of any stationary points using differentiation.

Differentiating implicitly with respect to xx:
2p2x−2ydydx=0  ⟹  dydx=p2xy2p^2x - 2y\frac{dy}{dx} = 0 \implies \frac{dy}{dx} = \frac{p^2x}{y}
For stationary points, dydx=0  ⟹  x=0\frac{dy}{dx} = 0 \implies x = 0.
However, as shown in the previous step, when x=0x = 0, there are no real values for yy. Thus, there are no stationary points on C1C_1.

4. Find the equations of the asymptotes.

The asymptotes for the hyperbola x2−y2p2=1x^2 - \frac{y^2}{p^2} = 1 are given by replacing the constant 11 with 00:
x2−y2p2=0x^2 - \frac{y^2}{p^2} = 0
y2=p2x2  ⟹  y=px or y=−pxy^2 = p^2x^2 \implies y = px \text{ or } y = -px

5. Synthesize the findings to describe the sketch of C1.

Draw the coordinate axes and the two diagonal asymptotes y=pxy = px and y=−pxy = -px. Since p>1p > 1, these lines have a steeper gradient than y=xy=x.
Draw the two branches of the curve opening outwards to the left and right:

  • The left branch has a vertex at (−1,0)(-1, 0).
  • The right branch has a vertex at (1,0)(1, 0).
    The curves must smoothly approach the asymptotes as x→±∞x \to \pm\infty.
答案

Intersection with axes: (1,0)(1, 0) and (−1,0)(-1, 0)
Stationary points: None
Asymptotes: y=pxy = px and y=−pxy = -px

(Sketch: A hyperbola centred at the origin with a left branch passing through (−1,0)(-1, 0) and a right branch passing through (1,0)(1, 0). Both branches approach the asymptotes y=pxy = px and y=−pxy = -px, which are steeper than y=±xy=\pm x since p>1p>1.)

7((ii))

1. Determine the equation of $C_2$ based on the change in the line of symmetry.

The curve C1C_1, given by p2x2−y2=p2p^2x^2 - y^2 = p^2, is centered at the origin and has a vertical line of symmetry at x=0x = 0.
For C2C_2 to have a line of symmetry at x=2x = 2 through a single transformation (based on standard transformations y=f(x+a)y = \mathrm{f}(x+a)), C1C_1 must be translated 2 units in the positive xx-direction.
Replacing xx with (x−2)(x - 2) yields the equation of C2C_2:
p2(x−2)2−y2=p2p^2(x - 2)^2 - y^2 = p^2

2. Substitute the given point $(4,3)$ to solve for $p$.

Substitute x=4x = 4 and y=3y = 3 into the equation of C2C_2:
p2(4−2)2−32=p2p^2(4 - 2)^2 - 3^2 = p^2
p2(2)2−9=p2p^2(2)^2 - 9 = p^2
4p2−9=p24p^2 - 9 = p^2
3p2=93p^2 = 9
p2=3p^2 = 3
Since it is given that p>1p > 1, we reject the negative root (p=−3p = -\sqrt{3}) to get:
p=3p = \sqrt{3}

答案

p=3p = \sqrt{3}

7((iii))

1. Identify the key features to sketch the graph of $y = f'(x)$.

Based on the given graph of y=f(x)y = f(x):

  • f(x)f(x) has a maximum at x=−1x = -1, meaning its gradient is zero. The graph of f′(x)f'(x) must pass through the xx-axis at (−1,0)(-1, 0).
  • For x<−1x < -1, f(x)f(x) is increasing, so f′(x)>0f'(x) > 0 (the graph is above the xx-axis).
  • For x>−1x > -1, f(x)f(x) is decreasing, so f′(x)<0f'(x) < 0 (the graph is below the xx-axis).
  • As x→∞x \to \infty, f(x)f(x) approaches a horizontal asymptote y=0y = 0. The gradient approaches 00, so f′(x)→0f'(x) \to 0 from below the xx-axis.

2. Relate the given equation to the graphs of $y = f'(x)$ and $C_1$.

We are given the equation:
p2x2−[f′(x)]2=p2p^2x^2 - [f'(x)]^2 = p^2
Substituting y=f′(x)y = f'(x), this can be rewritten as:
p2x2−y2=p2p^2x^2 - y^2 = p^2
This is exactly the equation of the hyperbola C1C_1. Thus, the roots of the given equation are exactly the xx-coordinates of the points of intersection between the graph of y=f′(x)y = f'(x) and the curve C1C_1.

3. Determine the number of intersections between the two graphs.

By visualizing both graphs on the same diagram:

  • At x=−1x = -1: The left branch of C1C_1 has a vertex at (−1,0)(-1, 0). The graph of f′(x)f'(x) also passes through (−1,0)(-1, 0). This gives 1 point of intersection.
  • For x<−1x < -1: f′(x)>0f'(x) > 0. The upper left branch of C1C_1 increases from y=0y=0 towards +∞+\infty with an initially infinite gradient, remaining above f′(x)f'(x). There is no intersection here.
  • For −1<x<1-1 < x < 1: C1C_1 is undefined (no real points), so there are no intersections.
  • For x≥1x \ge 1: The lower right branch of C1C_1 starts at (1,0)(1, 0) and decreases to −∞-\infty. At x=1x = 1, f′(1)<0f'(1) < 0 (meaning f′(x)f'(x) is below C1C_1). As x→∞x \to \infty, f′(x)f'(x) approaches 00 and stays bounded, while C1C_1 goes to −∞-\infty. Because f′(x)f'(x) starts below C1C_1 and ends up above it, the two curves must cross. This gives 1 point of intersection.

Total number of intersection points =1+1=2= 1 + 1 = 2.

答案

22

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