It is given that the curve CC has equation y=−4x2−6x−112x+2y = \dfrac{-4x^2 - 6x - 11}{2x + 2}, x∈Rx \in \mathbb{R}, x≠−1x \ne -1.

  1. (i)

    Without using a calculator, find the set of values that yy cannot take.

  2. (ii)

    Sketch CC, clearly stating the equations of any asymptotes, the coordinates of the stationary points and the point(s) where the curve crosses the axes.

  3. (iii)

    Deduce the range of values of aa such that the equation

    (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

    has no root, where aa is a positive constant.

  4. (iv)

    Describe fully a sequence of transformations which transform the curve y=2x+92xy = 2x + \dfrac{9}{2x} onto the curve y=−4x2−6x−112x+2y = \dfrac{-4x^2 - 6x - 11}{2x + 2}.

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Worked solution

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

Answer

−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.
Answer

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}.
Answer

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.
Answer
  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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