On the same axes, sketch the graphs of y=1x−2y = \frac{1}{x - 2} and y=4x−2y = 4x - 2, indicating clearly the equations of the asymptotes and the coordinates of the axial intercepts. Hence, or otherwise, solve the inequality 1x−2≤4x−2\frac{1}{x - 2} \le 4x - 2.

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Worked solution

1(i)

1. Find the asymptotes and axial intercepts of the rational function $y = \frac{1}{x - 2}$.

For y=1x−2y = \frac{1}{x - 2}:

  • The denominator cannot be zero, giving a vertical asymptote at x=2x = 2.
  • As x→±∞x \to \pm\infty, y→0y \to 0, giving a horizontal asymptote at y=0y = 0 (the xx-axis).
  • When x=0x = 0, y=10−2=−12y = \frac{1}{0 - 2} = -\frac{1}{2}. The yy-intercept is (0,−12)(0, -\frac{1}{2}).
  • There is no xx-intercept since the numerator is a non-zero constant (1≠01 \neq 0).

2. Find the axial intercepts of the linear function $y = 4x - 2$.

For y=4x−2y = 4x - 2:

  • When x=0x = 0, y=4(0)−2=−2y = 4(0) - 2 = -2. The yy-intercept is (0,−2)(0, -2).
  • When y=0y = 0, 4x−2=0  ⟹  x=124x - 2 = 0 \implies x = \frac{1}{2}. The xx-intercept is (12,0)(\frac{1}{2}, 0).

3. Sketch both graphs on the same axes.

On a single set of axes:

  • Draw a dashed vertical line labelled x=2x = 2.
  • Draw the two branches of the hyperbola for y=1x−2y = \frac{1}{x - 2}, labelling the intercept (0,−12)(0, -\frac{1}{2}).
  • Draw a straight line for y=4x−2y = 4x - 2, indicating the intercepts (0,−2)(0, -2) and (12,0)(\frac{1}{2}, 0).
Answer

A single pair of axes containing:

  • The curve y=1x−2y = \frac{1}{x - 2} with a dashed vertical asymptote x=2x = 2 and a horizontal asymptote y=0y = 0, passing through the yy-intercept (0,−12)(0, -\frac{1}{2}).
  • The straight line y=4x−2y = 4x - 2 passing through intercepts (0,−2)(0, -2) and (12,0)(\frac{1}{2}, 0).

1(ii)

1. Find the $x$-coordinates of the points of intersection between the curve and the line.

Equating the two expressions:
1x−2=4x−2\frac{1}{x - 2} = 4x - 2
1=(4x−2)(x−2)1 = (4x - 2)(x - 2)
1=4x2−10x+41 = 4x^2 - 10x + 4
4x2−10x+3=04x^2 - 10x + 3 = 0
Using the quadratic formula:
x=−(−10)±(−10)2−4(4)(3)2(4)x = \frac{-(-10) \pm \sqrt{(-10)^2 - 4(4)(3)}}{2(4)}
x=10±528=5±134x = \frac{10 \pm \sqrt{52}}{8} = \frac{5 \pm \sqrt{13}}{4}

2. Use the graphs from part (i) to deduce the intervals where the curve is below or equal to the line.

The inequality 1x−2≤4x−2\frac{1}{x - 2} \le 4x - 2 corresponds to the regions where the curve y=1x−2y = \frac{1}{x - 2} lies below or exactly on the line y=4x−2y = 4x - 2.

From the graphs sketched in (i), taking into account the relative positions of the curve and the line alongside the vertical asymptote at x=2x = 2:

  • For x<2x < 2, the curve is below the line starting from the first intersection point x=5−134x = \frac{5 - \sqrt{13}}{4} up to the vertical asymptote (where it tends towards −∞-\infty). Note that xx cannot be exactly 2.
  • For x>2x > 2, the curve emerges from +∞+\infty and drops below the line starting from the second intersection point x=5+134x = \frac{5 + \sqrt{13}}{4} onwards.

Thus, the combined solution is:
5−134≤x<2orx≥5+134\frac{5 - \sqrt{13}}{4} \le x < 2 \quad \text{or} \quad x \ge \frac{5 + \sqrt{13}}{4}

Answer

5−134≤x<2orx≥5+134\frac{5 - \sqrt{13}}{4} \le x < 2 \quad \text{or} \quad x \ge \frac{5 + \sqrt{13}}{4}

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