已知曲线 CC 的方程为 y=−4x2−6x−112x+2y = \dfrac{-4x^2 - 6x - 11}{2x + 2},其中 x∈Rx \in \mathbb{R} 且 x≠−1x \ne -1。

  1. (i)

    不使用计算器,求 yy 不能取的值的集合。

  2. (ii)

    画出 CC 的草图,明确指出所有渐近线的方程、驻点的坐标以及曲线与坐标轴的交点。

  3. (iii)

    推导 aa 的取值范围,使得方程

    (x+1)2(2x+2)2−a2(−4x2−8x−13)2=a2(2x+2)2(x+1)^2(2x+2)^2 - a^2\left(-4x^2 - 8x - 13\right)^2 = a^2(2x+2)^2

    没有根,其中 aa 是正常数。

  4. (iv)

    完整地描述一系列变换,这些变换将曲线 y=2x+92xy = 2x + \dfrac{9}{2x} 变换为曲线 y=−4x2−6x−112x+2y = \dfrac{-4x^2 - 6x - 11}{2x + 2}。

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完整解法

10(i)

1. Rearrange the given equation to form a quadratic equation in $x$.

y=−4x2−6x−112x+2y = \frac{-4x^2 - 6x - 11}{2x + 2}
y(2x+2)=−4x2−6x−11y(2x + 2) = -4x^2 - 6x - 11
2xy+2y=−4x2−6x−112xy + 2y = -4x^2 - 6x - 11
4x2+(2y+6)x+(2y+11)=04x^2 + (2y + 6)x + (2y + 11) = 0

2. Apply the discriminant condition for real roots to establish an inequality in $y$.

For xx to have real values, the discriminant must be non-negative (b2−4ac≥0b^2 - 4ac \ge 0):
(2y+6)2−4(4)(2y+11)≥0(2y + 6)^2 - 4(4)(2y + 11) \ge 0

3. Expand and simplify the discriminant inequality.

4y2+24y+36−16(2y+11)≥04y^2 + 24y + 36 - 16(2y + 11) \ge 0
4y2+24y+36−32y−176≥04y^2 + 24y + 36 - 32y - 176 \ge 0
4y2−8y−140≥04y^2 - 8y - 140 \ge 0
y2−2y−35≥0y^2 - 2y - 35 \ge 0

4. Factorise and solve for the valid range of $y$, then state the set of values $y$ cannot take.

(y−7)(y+5)≥0(y - 7)(y + 5) \ge 0
Hence, the possible values for yy are y≤−5y \le -5 or y≥7y \ge 7.
The set of values that yy cannot take is the complement of this range:
−5<y<7-5 < y < 7

答案

−5<y<7-5 < y < 7

10(ii)

1. Determine the equations of the asymptotes.

By algebraic division, rewrite the rational function:
y=−4x2−6x−112x+2=(−2x−1)(2x+2)−92x+2=−2x−1−92x+2y = \frac{-4x^2 - 6x - 11}{2x + 2} = \frac{(-2x - 1)(2x + 2) - 9}{2x + 2} = -2x - 1 - \frac{9}{2x + 2}
The vertical asymptote occurs when the denominator is zero:
2x+2=0  ⟹  x=−12x + 2 = 0 \implies x = -1
As x→±∞x \to \pm\infty, the fractional term 92x+2→0\frac{9}{2x+2} \to 0, giving the oblique asymptote:
y=−2x−1y = -2x - 1

2. Find the coordinates of the stationary points.

Differentiate yy with respect to xx to find the stationary points:
dydx=ddx(−2x−1−9(2x+2)−1)=−2+18(2x+2)2\frac{dy}{dx} = \frac{d}{dx}\left(-2x - 1 - 9(2x+2)^{-1}\right) = -2 + \frac{18}{(2x+2)^2}
Set dydx=0\frac{dy}{dx} = 0:
18(2x+2)2=2  ⟹  (2x+2)2=9\frac{18}{(2x+2)^2} = 2 \implies (2x+2)^2 = 9
2x+2=3or2x+2=−32x+2 = 3 \quad \text{or} \quad 2x+2 = -3
x=0.5orx=−2.5x = 0.5 \quad \text{or} \quad x = -2.5
Substitute the xx-values back into the equation of CC:
When x=0.5x = 0.5, y=−2(0.5)−1−93=−5y = -2(0.5) - 1 - \frac{9}{3} = -5.
When x=−2.5x = -2.5, y=−2(−2.5)−1−9−3=7y = -2(-2.5) - 1 - \frac{9}{-3} = 7.
The stationary points are (0.5,−5)(0.5, -5) and (−2.5,7)(-2.5, 7).

(Note: Students can also deduce the yy-values −5-5 and 77 from the boundary values established in part (i), then solve for the corresponding xx-values.)

3. Find the point(s) where the curve crosses the axes.

Substitute x=0x = 0 to find the yy-intercept:
y=−112=−5.5y = \frac{-11}{2} = -5.5
The curve crosses the yy-axis at (0,−5.5)(0, -5.5).

Substitute y=0y = 0 to find any xx-intercepts:
−4x2−6x−11=0-4x^2 - 6x - 11 = 0
Check the discriminant Δ\Delta:
Δ=(−6)2−4(−4)(−11)=36−176=−140\Delta = (-6)^2 - 4(-4)(-11) = 36 - 176 = -140
Since Δ<0\Delta < 0, there are no real solutions, hence the curve does not cross the xx-axis.

4. Sketch the curve $C$.

Draw the coordinate axes and plot the key features:

  • Draw and label the dashed lines for the asymptotes x=−1x = -1 and y=−2x−1y = -2x - 1.
  • Plot the local maximum point at (0.5,−5)(0.5, -5) and the local minimum point at (−2.5,7)(-2.5, 7).
  • Plot the yy-intercept at (0,−5.5)(0, -5.5).
  • Sketch the two continuous branches of the rational function approaching the asymptotes smoothly.
答案

Asymptotes: x=−1,y=−2x−1x = -1, y = -2x - 1
Stationary points: (−2.5,7)(-2.5, 7) and (0.5,−5)(0.5, -5)
Axial intercept: (0,−5.5)(0, -5.5)

(A sketch of the curve showing the two branches, bounded by the stated asymptotes, with the turning points and yy-intercept clearly marked.)

10(iii)

1. Apply a substitution to simplify the given equation.

Let X=x+1X = x + 1. Then 2x+2=2(x+1)=2X2x + 2 = 2(x + 1) = 2X.
By completing the square for the expression −4x2−8x−13-4x^2 - 8x - 13:
−4(x2+2x)−13=−4(x2+2x+1)−9=−4X2−9-4(x^2 + 2x) - 13 = -4(x^2 + 2x + 1) - 9 = -4X^2 - 9
Substitute these into the given equation:
X2(2X)2−a2(−4X2−9)2=a2(2X)2X^2(2X)^2 - a^2(-4X^2 - 9)^2 = a^2(2X)^2

2. Expand and simplify the equation into a polynomial in $X$.

Expand the terms:
4X4−a2(16X4+72X2+81)=4a2X24X^4 - a^2(16X^4 + 72X^2 + 81) = 4a^2X^2
4X4−16a2X4−72a2X2−81a2−4a2X2=04X^4 - 16a^2X^4 - 72a^2X^2 - 81a^2 - 4a^2X^2 = 0
Group the terms by powers of XX:
4(1−4a2)X4−76a2X2−81a2=04(1 - 4a^2)X^4 - 76a^2X^2 - 81a^2 = 0

3. Analyze the roots of the quadratic in $X^2$ to find the condition for no real roots.

Let Z=X2Z = X^2. Since x≠−1x \ne -1, we require Z>0Z > 0 for xx to be real.
The equation is a quadratic in ZZ:
4(1−4a2)Z2−76a2Z−81a2=04(1 - 4a^2)Z^2 - 76a^2Z - 81a^2 = 0
For the original equation to have no root, this quadratic must have no positive real root for ZZ. Notice that since a>0a > 0, the coefficients −76a2-76a^2 and −81a2-81a^2 are both strictly negative.

  • If 1−4a2>01 - 4a^2 > 0, the signs of the coefficients are (+,−,−)(+, -, -), meaning the product of roots is negative, so there is exactly one positive real root.
  • If 1−4a2≤01 - 4a^2 \le 0, all coefficients are ≤0\le 0. For any Z>0Z > 0, the polynomial's value is strictly negative and can never equal 00, so there are no positive real roots.
    Thus, for no root, we must have:
    1−4a2≤0  ⟹  a2≥141 - 4a^2 \le 0 \implies a^2 \ge \frac{1}{4}
    Since aa is a positive constant, a≥12a \ge \frac{1}{2}.
答案

a≥12a \ge \frac{1}{2}

10(iv)

1. Express the given rational function in quotient-remainder form.

y=−4x2−6x−112x+2y = \frac{-4x^2 - 6x - 11}{2x + 2}
By algebraic manipulation or polynomial long division:
y=−2x(2x+2)−2x−112x+2y = \frac{-2x(2x + 2) - 2x - 11}{2x + 2}
y=−2x+−(2x+2)−92x+2y = -2x + \frac{-(2x + 2) - 9}{2x + 2}
y=−2x−1−92x+2y = -2x - 1 - \frac{9}{2x + 2}

2. Relate the simplified expression to the original function.

Let f(x)=2x+92xf(x) = 2x + \frac{9}{2x}. We want to express the simplified yy in terms of f(x)f(x).
Rewrite yy by completing the form of f(x+1)f(x+1):
y=−(2x+1+92(x+1))y = -\left(2x + 1 + \frac{9}{2(x+1)}\right)
y=−(2x+2+92(x+1)−1)y = -\left(2x + 2 + \frac{9}{2(x+1)} - 1\right)
y=−(2(x+1)+92(x+1))+1y = -\left(2(x+1) + \frac{9}{2(x+1)}\right) + 1
y=−f(x+1)+1y = -f(x+1) + 1

3. State the sequence of transformations.

The equation y=−f(x+1)+1y = -f(x+1) + 1 can be mapped from y=f(x)y = f(x) by:

  1. A reflection in the xx-axis, transforming y=f(x)y = f(x) into y=−f(x)y = -f(x).
  2. A translation by the vector (−11)\begin{pmatrix} -1 \\ 1 \end{pmatrix}, transforming y=−f(x)y = -f(x) into y−1=−f(x−(−1))  ⟹  y=−f(x+1)+1y - 1 = -f(x - (-1)) \implies y = -f(x+1) + 1.
答案
  1. Reflection in the xx-axis.
  2. Translation by the vector (−11)\begin{pmatrix} -1 \\ 1 \end{pmatrix}.

(Note: Other equivalent sequences of transformations are acceptable)

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