The curve CC has equation given by y=1+3x−18x29x+3y = \dfrac{1+3x-18x^2}{9x+3}.

  1. (a)

    Without the use of a calculator, find the range of values that yy can take.

  2. (b)

    Sketch the graph of CC indicating clearly its asymptotes and stationary points.

  3. (c)

    State a sequence of transformations that will transform the curve CC to the curve with equation y=1+3(2x−1)−18(2x−1)29(2x−1)+3y = \dfrac{1+3(2x-1)-18(2x-1)^2}{9(2x-1)+3}.

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Worked solution

5(a)

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

y=1+3x−18x29x+3y = \frac{1+3x-18x^2}{9x+3}
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 condition for real roots to form an inequality in $y$.

For xx to be real, the discriminant of the quadratic equation must be non-negative (Δ≥0\Delta \ge 0):
b2−4ac≥0b^2 - 4ac \ge 0
(9y−3)2−4(18)(3y−1)≥0(9y-3)^2 - 4(18)(3y-1) \ge 0

3. Simplify the inequality without a calculator by extracting common factors.

9(3y−1)2−72(3y−1)≥09(3y-1)^2 - 72(3y-1) \ge 0
Factorising out the common term 9(3y−1)9(3y-1):
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
Dividing by 27:
(3y−1)(y−3)≥0(3y-1)(y-3) \ge 0

4. Solve the quadratic inequality to determine the valid range of $y$.

Solving (3y−1)(y−3)≥0(3y-1)(y-3) \ge 0, the values of yy lie outside the roots 13\frac{1}{3} and 33:
y≤13 or y≥3y \le \frac{1}{3} \text{ or } y \ge 3

Answer

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

5(b)

1. Find the equations of the asymptotes using algebraic long division.

y=1+3x−18x29x+3y = \frac{1+3x-18x^2}{9x+3}
By performing algebraic long division, we can rewrite the rational function as:
y=−2x+1−29x+3y = -2x + 1 - \frac{2}{9x+3}
From the denominator, the vertical asymptote is 9x+3=0  ⟹  x=−139x + 3 = 0 \implies x = -\frac{1}{3}.
From the linear quotient, the oblique (slant) asymptote is y=−2x+1y = -2x + 1.

2. Determine the coordinates of the stationary points and axial intercepts.

Using a graphing calculator (or by setting dydx=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0), we locate the stationary points. Using the range derived in part (a), the turning points occur at y=13y = \frac{1}{3} and y=3y = 3.
The local maximum point is (0,13)(0, \frac{1}{3}).
The local minimum point is (−23,3)(-\frac{2}{3}, 3).
To find the xx-intercepts, set y=0y = 0:
1+3x−18x2=01+3x-18x^2 = 0
(1+6x)(1−3x)=0  ⟹  x=−16 or x=13(1+6x)(1-3x) = 0 \implies x = -\frac{1}{6} \text{ or } x = \frac{1}{3}

3. Sketch the graph showing all key features.

Draw the asymptotes x=−13x = -\frac{1}{3} and y=−2x+1y = -2x + 1 with dotted lines.
Sketch the upper branch containing the minimum point (−23,3)(-\frac{2}{3}, 3), staying bounded above y=−2x+1y = -2x + 1 as x→−∞x \to -\infty.
Sketch the lower branch containing the maximum point (0,13)(0, \frac{1}{3}) (which is also the yy-intercept), passing through the xx-intercepts (−16,0)(-\frac{1}{6}, 0) and (13,0)(\frac{1}{3}, 0), and bounded below y=−2x+1y = -2x + 1 as x→∞x \to \infty.

Answer

[A valid sketch must be drawn on Cartesian axes showing the following features]

  • Vertical Asymptote: x=−13x = -\frac{1}{3}
  • Oblique Asymptote: y=−2x+1y = -2x + 1
  • Stationary Points: Local minimum at (−23,3)(-\frac{2}{3}, 3) and Local maximum at (0,13)(0, \frac{1}{3})
  • Intercepts: xx-intercepts at (−16,0)(-\frac{1}{6}, 0) and (13,0)(\frac{1}{3}, 0); yy-intercept at (0,13)(0, \frac{1}{3})

5(c)

1. Express the new equation in terms of the original function.

Let f(x)=1+3x−18x29x+3f(x) = \dfrac{1+3x-18x^2}{9x+3} be the equation of curve CC.

The target equation is 1+3(2x−1)−18(2x−1)29(2x−1)+3\dfrac{1+3(2x-1)-18(2x-1)^2}{9(2x-1)+3}.
This is formed by replacing every xx in f(x)f(x) with (2x−1)(2x-1). Hence, the new equation is y=f(2x−1)y = f(2x-1).

2. State the sequence of transformations that maps $f(x)$ to $f(2x-1)$.

We can build f(2x−1)f(2x-1) through the following sequence:

  1. Replace xx with x−1x-1: This corresponds to a translation of 11 unit in the positive xx-direction, transforming y=f(x)y = f(x) to y=f(x−1)y = f(x-1).
  2. Replace xx with 2x2x: This corresponds to a scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2}, transforming y=f(x−1)y = f(x-1) to y=f(2x−1)y = f(2x-1).

(Note: An alternative valid sequence is a scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2} to get f(2x)f(2x), followed by a translation of 12\frac{1}{2} units in the positive xx-direction to get f(2(x−12))=f(2x−1)f(2(x-\frac{1}{2})) = f(2x-1).)

Answer
  1. A translation of 11 unit in the positive xx-direction (or by vector (10)\begin{pmatrix} 1 \\ 0 \end{pmatrix}).
  2. A scaling parallel to the xx-axis by a scale factor of 12\frac{1}{2}.

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