曲线 CC 的方程由 y=1+3x−18x29x+3y = \frac{1+3x-18x^{2}}{9x+3} 给出。

  1. (a)

    在不使用计算器的情况下,求 yy 的取值范围。

  2. (b)

    草拟曲线 CC 的图像,并清晰地标出其渐近线和驻点。

  3. (c)

    写出一系列能将曲线 CC 变换为方程为 y=1+3(2x−1)−18(2x−1)29(2x−1)+3y = \frac{1+3(2x-1)-18(2x-1)^2}{9(2x-1)+3} 的曲线的变换步骤。

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完整解法

5(a)

1. Rearrange the equation of the curve into a quadratic equation in $x$.

y(9x+3)=1+3x−18x2y(9x+3) = 1+3x-18x^2
9xy+3y=1+3x−18x29xy + 3y = 1 + 3x - 18x^2
18x2+(9y−3)x+(3y−1)=018x^2 + (9y-3)x + (3y-1) = 0

2. Apply the discriminant condition for real roots.

For yy to be in the range, there must be at least one real value of xx that corresponds to it. Hence, the discriminant of the quadratic equation in xx must be non-negative (Δ≥0\Delta \ge 0).
(9y−3)2−4(18)(3y−1)≥0(9y-3)^2 - 4(18)(3y-1) \ge 0

3. Simplify the discriminant inequality.

Factorising to simplify the expansion:
9(3y−1)2−72(3y−1)≥09(3y-1)^2 - 72(3y-1) \ge 0
9(3y−1)[(3y−1)−8]≥09(3y-1)[(3y-1) - 8] \ge 0
9(3y−1)(3y−9)≥09(3y-1)(3y-9) \ge 0
27(3y−1)(y−3)≥027(3y-1)(y-3) \ge 0
(3y−1)(y−3)≥0(3y-1)(y-3) \ge 0

4. Solve the quadratic inequality to find the range.

The critical values are y=13y = \frac{1}{3} and y=3y = 3. Since the quadratic expression represents a 'U-shaped' parabola and is ≥0\ge 0, we take the regions outside the roots:
y≤13 or y≥3y \le \frac{1}{3} \text{ or } y \ge 3

答案

y≤13 or y≥3y \le \frac{1}{3} \text{ or } y \ge 3

5(b)

1. Find the equations of the asymptotes by rewriting the rational function.

y=−18x2+3x+19x+3=−2x(9x+3)+9x+19x+3=−2x+1(9x+3)−29x+3=−2x+1−29x+3\begin{aligned} y &= \frac{-18x^2+3x+1}{9x+3} \\ &= \frac{-2x(9x+3) + 9x+1}{9x+3} \\ &= -2x + \frac{1(9x+3)-2}{9x+3} \\ &= -2x + 1 - \frac{2}{9x+3} \end{aligned}
The vertical asymptote is x=−13x = -\frac{1}{3} and the oblique asymptote is y=−2x+1y = -2x + 1.

2. Find the exact coordinates of the stationary points using the range of $y$ found in part (a).

From part (a), the range is y≤13y \le \frac{1}{3} or y≥3y \ge 3. The boundary values give the yy-coordinates of the stationary points.
Using the rearranged equation from part (a): 18x2+(9y−3)x+(3y−1)=018x^2 + (9y - 3)x + (3y - 1) = 0
Substitute y=13y = \frac{1}{3}:
18x2=0  ⟹  x=018x^2 = 0 \implies x = 0
Substitute y=3y = 3:
18x2+24x+8=0  ⟹  2(3x+2)2=0  ⟹  x=−2318x^2 + 24x + 8 = 0 \implies 2(3x+2)^2 = 0 \implies x = -\frac{2}{3}
The stationary points are (0,13)(0, \frac{1}{3}) and (−23,3)(-\frac{2}{3}, 3).

3. Determine axial intercepts and produce the sketch.

xx-intercepts occur when y=0y = 0:
1+3x−18x2=0  ⟹  (1−3x)(1+6x)=0  ⟹  x=13 or −161+3x-18x^2 = 0 \implies (1-3x)(1+6x) = 0 \implies x = \frac{1}{3} \text{ or } -\frac{1}{6}.
yy-intercept is at x=0  ⟹  y=13x=0 \implies y=\frac{1}{3} (which coincides with the local maximum point).
The sketch should clearly display two branches of a hyperbola confined by the asymptotes x=−13x=-\frac{1}{3} and y=−2x+1y=-2x+1, with the right branch having a maximum at (0,13)(0, \frac{1}{3}) and the left branch having a minimum at (−23,3)(-\frac{2}{3}, 3).

答案

Asymptotes: x=−13x = -\frac{1}{3}, y=−2x+1y = -2x + 1
Stationary points: Maximum at (0,13)(0, \frac{1}{3}), Minimum at (−23,3)(-\frac{2}{3}, 3)
(A sketch showing these features, along with xx-intercepts at 13\frac{1}{3} and −16-\frac{1}{6}, is drawn.)

5(c)

1. Identify the relationship between the new equation and curve $C$, and state the first transformation.

Let the equation of curve CC be y=f(x)=1+3x−18x29x+3y = \mathrm{f}(x) = \frac{1+3x-18x^{2}}{9x+3}.
The new equation is obtained by replacing every xx with 2x−12x - 1, so it is given by y=f(2x−1)y = \mathrm{f}(2x - 1).
The first transformation is a translation of 11 unit in the positive xx-direction, which transforms y=f(x)y = \mathrm{f}(x) to y=f(x−1)y = \mathrm{f}(x - 1).

2. State the second transformation to achieve the final equation.

The second transformation is a scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2}.
This replaces xx with 2x2x, transforming y=f(x−1)y = \mathrm{f}(x - 1) to y=f(2x−1)y = \mathrm{f}(2x - 1).

答案
  1. Translation of 11 unit in the positive xx-direction.
  2. Scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2}.

(Alternatively: 1. Scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2}. 2. Translation of 12\frac{1}{2} unit in the positive xx-direction.)

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