曲线 CC 的方程为 y=x2−3x+31−xy = \dfrac{x^{2}-3x+3}{1-x}。

  1. (a)

    画出 CC 的草图,指出任何极值点以及与坐标轴交点的坐标,并写出所有渐近线的方程。

  2. (b)

    确定 kk 的确切值集合,使得对于所有实数 xx, CC 与直线 y=kxy = kx 都有两个交点。

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完整解法

2(a)

1. Find the point(s) of intersection with the coordinate axes.

Substitute x=0x = 0 into the equation of CC:
y=02−3(0)+31−0=3y = \frac{0^2 - 3(0) + 3}{1 - 0} = 3
The yy-intercept is (0,3)(0, 3).

Substitute y=0y = 0:
x2−3x+3=0x^2 - 3x + 3 = 0
The discriminant Δ=(−3)2−4(1)(3)=9−12=−3\Delta = (-3)^2 - 4(1)(3) = 9 - 12 = -3.
Since Δ<0\Delta < 0, there are no real roots, hence the curve does not intersect the xx-axis.

2. Determine the equations of the asymptotes.

The denominator cannot be zero, giving the vertical asymptote:
1−x=0  ⟹  x=11 - x = 0 \implies x = 1

To find the oblique asymptote, perform polynomial division on x2−3x+31−x\frac{x^2 - 3x + 3}{1-x}:
x2−3x+3−x+1=−x(1−x)−2x+31−x=−x+−2(1−x)+11−x=−x+2+11−x\frac{x^2 - 3x + 3}{-x + 1} = \frac{-x(1-x) - 2x + 3}{1-x} = -x + \frac{-2(1-x) + 1}{1-x} = -x + 2 + \frac{1}{1-x}
As x→±∞x \to \pm\infty, 11−x→0\frac{1}{1-x} \to 0, so the oblique asymptote is:
y=−x+2y = -x + 2

3. Find the coordinates of the turning points.

Differentiate y=−x+2+(1−x)−1y = -x + 2 + (1-x)^{-1} with respect to xx:
dydx=−1+(1−x)−2(−1)(−1)=−1+1(1−x)2\frac{dy}{dx} = -1 + (1-x)^{-2}(-1)(-1) = -1 + \frac{1}{(1-x)^2}
At the turning points, dydx=0\frac{dy}{dx} = 0:
−1+1(1−x)2=0  ⟹  (1−x)2=1-1 + \frac{1}{(1-x)^2} = 0 \implies (1-x)^2 = 1
1−x=1or1−x=−11 - x = 1 \quad \text{or} \quad 1 - x = -1
x=0orx=2x = 0 \quad \text{or} \quad x = 2
Substitute back to find the corresponding yy-values:
When x=0x = 0, y=−0+2+11−0=3y = -0 + 2 + \frac{1}{1-0} = 3.
When x=2x = 2, y=−2+2+11−2=−1y = -2 + 2 + \frac{1}{1-2} = -1.

The turning points are (0,3)(0, 3) and (2,−1)(2, -1).

4. Describe the sketch of the curve $C$.

Draw a set of axes and include the following:

  • A vertical dashed line at x=1x = 1.
  • A dashed line for the oblique asymptote y=−x+2y = -x + 2.
  • The upper branch for x<1x < 1, passing through the local minimum and yy-intercept at (0,3)(0, 3), bounded by the asymptotes.
  • The lower branch for x>1x > 1, with a local maximum at (2,−1)(2, -1), bounded by the asymptotes.
答案

Turning points: (0,3)(0, 3) and (2,−1)(2, -1)
Intersection with axes: (0,3)(0, 3) only
Asymptotes: x=1x = 1 and y=−x+2y = -x + 2
(A complete sketch should be drawn on the coordinate axes incorporating these features.)

2(b)

1. Form a quadratic equation in $x$ for the intersection

At intersection points kx=x2−3x+31−xkx = \dfrac{x^{2}-3x+3}{1-x}.

Multiply by (1−x)(1-x):
kx(1−x)=x2−3x+3kx(1-x) = x^{2}-3x+3
kx−kx2=x2−3x+3kx - kx^{2} = x^{2}-3x+3
Rearranging:
(1+k)x2−(3+k)x+3=0.(1+k)x^{2} - (3+k)x + 3 = 0.
For two points of intersection this quadratic must have two distinct real roots, so we also need 1+k≠01+k \neq 0.

2. Apply discriminant > 0 and solve the quadratic inequality in $k$

Two distinct real roots require discriminant >0> 0:
(3+k)2−4(1+k)(3)>0(3+k)^{2} - 4(1+k)(3) > 0
9+6k+k2−12−12k>09 + 6k + k^{2} - 12 - 12k > 0
k2−6k−3>0.k^{2} - 6k - 3 > 0.
Roots of k2−6k−3=0k^{2}-6k-3=0: k=6±36+122=3±23k = \dfrac{6 \pm \sqrt{36+12}}{2} = 3 \pm 2\sqrt{3}.

Since the parabola opens upward:
k<3−23ork>3+23.k < 3 - 2\sqrt{3} \quad \text{or} \quad k > 3 + 2\sqrt{3}.

3. Exclude the degenerate value and state the set

When 1+k=01+k = 0, i.e. k=−1k = -1, the equation becomes linear −2x+3=0-2x+3 = 0, giving only ONE intersection. Since −1<3−23 (≈−0.46)-1 < 3-2\sqrt{3}\ (\approx -0.46), k=−1k=-1 lies in the region above and must be removed.

Hence
{ k∈R:k<3−23 or k>3+23 }∖{−1}.\{\,k \in \mathbb{R}: k < 3 - 2\sqrt{3} \text{ or } k > 3 + 2\sqrt{3}\,\} \setminus \{-1\}.

答案

{ k∈R:k<3−23 or k>3+23 }∖{−1}\{\,k \in \mathbb{R}: k < 3 - 2\sqrt{3} \text{ or } k > 3 + 2\sqrt{3}\,\} \setminus \{-1\}

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