The curve CC has equation y=2+3ax−ay = 2 + \frac{3a}{x - a}, where aa is a positive constant.

  1. (a)

    Describe fully a sequence of transformations which would transform the curve y=1xy = \frac{1}{x} onto the curve CC.

  2. (b)

    Sketch the graph of y=∣2+3ax−a∣y = \left| 2 + \frac{3a}{x - a} \right|, labelling the equations of the asymptotes and coordinates of the axial intercepts.

  3. (c)

    Use an algebraic method to solve ∣2+3x−1∣=3\left| 2 + \frac{3}{x - 1} \right| = 3 and state the solution for ∣2+3x−1∣<3\left| 2 + \frac{3}{x - 1} \right| < 3.

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Worked solution

4(a)

1. Express the curve $C$ in terms of the base function $\mathrm{f}(x) = \frac{1}{x}$.

Let f(x)=1x\mathrm{f}(x) = \frac{1}{x}.
The equation of CC is y=3ax−a+2y = \frac{3a}{x-a} + 2.
This can be written as y=3af(x−a)+2y = 3a\mathrm{f}(x-a) + 2.

2. Identify the scaling transformation.

The factor of 3a3a corresponds to y=3af(x)y = 3a\mathrm{f}(x), which is a scaling parallel to the yy-axis by a scale factor of 3a3a.

3. Identify the translation and state the sequence.

The terms (x−a)(x-a) and +2+ 2 correspond to a translation of aa units in the positive xx-direction and 22 units in the positive yy-direction.
This is equivalent to a translation by the vector (a2)\begin{pmatrix} a \\ 2 \end{pmatrix}.
(Note: To achieve +2+2 strictly at the end, the scaling must precede the yy-translation. Thus, stating the scaling first followed by the full vector translation works perfectly).

Answer
  1. A scaling parallel to the yy-axis by a scale factor of 3a3a.
  2. A translation by the vector (a2)\begin{pmatrix} a \\ 2 \end{pmatrix}.

4(b)

1. Find the equations of the asymptotes.

For the original curve y=2+3ax−ay = 2 + \frac{3a}{x-a}, as x→±∞x \to \pm\infty, 3ax−a→0\frac{3a}{x-a} \to 0, giving a horizontal asymptote y=2y = 2.
The denominator is zero when x=ax = a, giving a vertical asymptote x=ax = a.
Applying the modulus function y=∣2+3ax−a∣y = \left| 2 + \frac{3a}{x-a} \right|, the asymptotes remain y=∣2∣=2y = |2| = 2 and x=ax = a.

2. Find the coordinates of the $x$- and $y$-intercepts.

At the xx-intercept, y=0y = 0:
2+3ax−a=02 + \frac{3a}{x-a} = 0
3ax−a=−2\frac{3a}{x-a} = -2
−2(x−a)=3a  ⟹  −2x+2a=3a  ⟹  x=−a2-2(x - a) = 3a \implies -2x + 2a = 3a \implies x = -\frac{a}{2}
The xx-intercept is (−a2,0)(-\frac{a}{2}, 0).

At the yy-intercept, x=0x = 0:
y=∣2+3a−a∣=∣2−3∣=∣−1∣=1y = \left| 2 + \frac{3a}{-a} \right| = |2 - 3| = |-1| = 1
The yy-intercept is (0,1)(0, 1).

3. Deduce the shape of the curve to sketch it.

For x>ax > a, 2+3ax−a>02 + \frac{3a}{x-a} > 0, so the curve approaches y=2y=2 from above and goes to +∞+\infty near x=ax=a.
For x<ax < a, the original function is negative between x=−a2x = -\frac{a}{2} and x=ax = a. The modulus reflects this portion in the xx-axis, so it turns upwards from (−a2,0)(-\frac{a}{2}, 0), passes through (0,1)(0, 1), and goes to +∞+\infty near x=ax=a.
For x<−a2x < -\frac{a}{2}, the original function is positive and approaches y=2y=2 from below as x→−∞x \to -\infty.

Answer

A curve drawn on Cartesian axes featuring:

  • Vertical asymptote at x=ax = a
  • Horizontal asymptote at y=2y = 2
  • xx-intercept at (−a2,0)(-\frac{a}{2}, 0)
  • yy-intercept at (0,1)(0, 1)
  • The branch for x>ax > a comes down from x=ax=a (at +∞+\infty) and approaches y=2y=2 from above as x→∞x \to \infty.
  • The branch for x<ax < a goes from approaching y=2y=2 from below as x→−∞x \to -\infty, meets the xx-axis at (−a2,0)(-\frac{a}{2}, 0), then reflects upwards through (0,1)(0, 1) to approach x=ax=a (at +∞+\infty).

4(c)

1. Set up the two cases to remove the modulus for the equation.

2+3x−1=3or2+3x−1=−32 + \frac{3}{x - 1} = 3 \quad \text{or} \quad 2 + \frac{3}{x - 1} = -3

2. Solve the first case.

3x−1=1x−1=3x=4\frac{3}{x - 1} = 1 \\ x - 1 = 3 \\ x = 4

3. Solve the second case.

3x−1=−5−5(x−1)=3−5x+5=35x=2x=25\frac{3}{x - 1} = -5 \\ -5(x - 1) = 3 \\ -5x + 5 = 3 \\ 5x = 2 \\ x = \frac{2}{5}

4. State the solution to the inequality based on the points found.

From the shape of the graph sketched in part (b) (with a=1a=1), the curve approaches the horizontal asymptote y=2y = 2 at the extremities, so it falls below y=3y = 3 in the outer regions bounded by the solutions.
x<25orx>4x < \frac{2}{5} \quad \text{or} \quad x > 4

Answer

x=4x = 4 or x=25x = \frac{2}{5}
x<25x < \frac{2}{5} or x>4x > 4

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