Sketch the graphs of y=1+a−2x−ay = 1 + \frac{a-2}{x-a} and y=−1ax+2ay = -\frac{1}{a}x + \frac{2}{a} on a single diagram, where aa is a positive constant and 1<a<21 < a < 2, showing all asymptotes and axial intercepts clearly.

  1. (i)

    Using the graphs, solve, in terms of aa, 1+a−2x−a>−1ax+2a1 + \frac{a-2}{x-a} > -\frac{1}{a}x + \frac{2}{a}.

  2. (ii)

    Hence, solve 1+ax−2x1−ax>1ax+2a1 + \frac{ax - 2x}{1 - ax} > \frac{1}{ax} + \frac{2}{a}.

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Worked solution

10(i)

1. Identify the $x$-coordinates of the intersections and the vertical asymptote.

From the equations given for the graphs, substituting x=0x=0 gives y=1+a−2−a=2ay = 1 + \frac{a-2}{-a} = \frac{2}{a} for the curve and y=2ay = \frac{2}{a} for the line.
Substituting y=0y=0 gives 0=1+a−2x−a  ⟹  x=20 = 1 + \frac{a-2}{x-a} \implies x = 2 for the curve and 0=−1ax+2a  ⟹  x=20 = -\frac{1}{a}x + \frac{2}{a} \implies x = 2 for the line.
Thus, the graphs intersect at the points (0,2a)(0, \frac{2}{a}) and (2,0)(2, 0), so the intersection xx-coordinates are x=0x=0 and x=2x=2.

The vertical asymptote of the curve y=1+a−2x−ay = 1 + \frac{a-2}{x-a} is x=ax = a.
Since 1<a<21 < a < 2, the points lie in the order x=0x=0, x=ax=a, and x=2x=2.

2. Solve the inequality by determining where the curve lies above the line.

The inequality 1+a−2x−a>−1ax+2a1 + \frac{a-2}{x-a} > -\frac{1}{a}x + \frac{2}{a} asks for the range of values of xx where the curve CC is strictly above the line LL.

Based on the branches of the hyperbola (since a−2<0a-2 < 0) and the line LL with negative gradient −1a-\frac{1}{a}:

  • For x<ax < a (left branch): The curve passes through (0,2a)(0, \frac{2}{a}) and tends to +∞+\infty as x→a−x \to a^-. It is above the line for 0<x<a0 < x < a.
  • For x>ax > a (right branch): The curve passes through (2,0)(2, 0) and tends to 11 as x→∞x \to \infty. It is above the line for x>2x > 2.

Hence, the required solution is 0<x<a or x>20 < x < a \text{ or } x > 2.

Answer

0<x<a or x>20 < x < a \text{ or } x > 2

10(ii)

1. Identify the substitution linking (ii) to (i)

Replace xx by 1x\dfrac{1}{x} in the inequality of part (i):
1+a−21x−a=1+(a−2)x1−ax=1+ax−2x1−ax,1+\frac{a-2}{\frac1x-a}=1+\frac{(a-2)x}{1-ax}=1+\frac{ax-2x}{1-ax},
which is exactly the left-hand side of (ii). The right-hand side −1ax+2a-\frac1a x+\frac2a becomes −1ax+2a-\frac{1}{ax}+\frac{2}{a}. So (ii) is just (i) with xx replaced by 1x\dfrac1x.

2. Transfer the solution of (i) using the substitution

From part (i) the inequality holds for 0<x<a0<x<a or x>2x>2. Replacing xx by 1x\dfrac1x, the inequality in (ii) holds when
0<1x<aor1x>2.0<\frac1x<a\qquad\text{or}\qquad \frac1x>2.

3. Solve the transformed inequalities and combine

For 0<1x<a0<\dfrac1x<a: 1x>0⇒x>0\dfrac1x>0\Rightarrow x>0, and 1x<a⇒x>1a\dfrac1x<a\Rightarrow x>\dfrac1a (since x>0x>0). This gives x>1ax>\dfrac1a.

For 1x>2\dfrac1x>2: this requires x>0x>0 and x<12x<\dfrac12, i.e. 0<x<120<x<\dfrac12.

Since 1<a<21<a<2 gives 12<1a<1\dfrac12<\dfrac1a<1, the two ranges are separate. Hence
0<x<12orx>1a.0<x<\frac12\quad\text{or}\quad x>\frac1a.

Answer

0<x<12orx>1a0 < x < \dfrac{1}{2}\quad\text{or}\quad x > \dfrac{1}{a}

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