A curve CC has equation y=2x2−5ax+2a2+1x−ay = \frac{2x^2 - 5ax + 2a^2 + 1}{x - a}, x≠ax \ne a.

  1. (i)

    Find the equations of the asymptotes, giving your answers in terms of aa.

  2. (ii)

    If curve CC has no stationary points, find the range of values of aa.

  3. (iii)

    Sketch curve CC for a>1a > 1, stating clearly the equations of the asymptotes and the yy-intercept of the curve only.

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Worked solution

2(i)

1. Find the equation of the vertical asymptote

The function is undefined when the denominator is zero.
x−a=0  ⟹  x=ax - a = 0 \implies x = a
The vertical asymptote is x=ax = a.

2. Perform algebraic manipulation or polynomial division to find the oblique asymptote

Rewrite the numerator to express yy in the form y=mx+c+kx−ay = mx + c + \frac{k}{x - a}:

y=2x2−5ax+2a2+1x−a=2x2−2ax−3ax+3a2−a2+1x−a=2x(x−a)−3a(x−a)+1−a2x−a=2x−3a+1−a2x−a\begin{aligned} y &= \frac{2x^2 - 5ax + 2a^2 + 1}{x - a} \\ &= \frac{2x^2 - 2ax - 3ax + 3a^2 - a^2 + 1}{x - a} \\ &= \frac{2x(x - a) - 3a(x - a) + 1 - a^2}{x - a} \\ &= 2x - 3a + \frac{1 - a^2}{x - a} \end{aligned}

As x→±∞x \to \pm\infty, the fraction 1−a2x−a→0\frac{1 - a^2}{x - a} \to 0.
Therefore, the oblique asymptote is y=2x−3ay = 2x - 3a.

Answer

x=ax = a and y=2x−3ay = 2x - 3a

2(ii)

1. Differentiate the curve equation using the simplified form obtained from finding the asymptotes.

From part (i), by performing algebraic division, the curve can be written as:
y=2x−3a+1−a2x−ay = 2x - 3a + \frac{1 - a^2}{x - a}

Differentiating with respect to xx:
dydx=2−1−a2(x−a)2\frac{dy}{dx} = 2 - \frac{1 - a^2}{(x - a)^2}

2. Equate the derivative to zero and rearrange to isolate $(x - a)^2$.

For stationary points, we set dydx=0\frac{dy}{dx} = 0:
2−1−a2(x−a)2=02 - \frac{1 - a^2}{(x - a)^2} = 0
1−a2(x−a)2=2\frac{1 - a^2}{(x - a)^2} = 2
(x−a)2=1−a22(x - a)^2 = \frac{1 - a^2}{2}

3. Apply the condition for the equation to have no real solutions and solve the inequality for $a$.

For the curve to have no stationary points, the equation must yield no valid real solutions for xx.
Since (x−a)2≥0(x - a)^2 \ge 0 for all real xx, there are no real solutions if the right-hand side is strictly negative:
1−a22<0\frac{1 - a^2}{2} < 0
1−a2<01 - a^2 < 0
a2>1a^2 > 1

Therefore, the range of values is:
a<−1 or a>1a < -1 \text{ or } a > 1

Answer

a<−1 or a>1a < -1 \text{ or } a > 1

2(iii)

1. Find the equations of the vertical and oblique asymptotes by rewriting the equation of the curve.

By algebraic division:
y=2x2−5ax+2a2+1x−a=2x(x−a)−3ax+2a2+1x−a=2x(x−a)−3a(x−a)−3a2+2a2+1x−a=2x−3a+1−a2x−a\begin{aligned} y &= \frac{2x^2 - 5ax + 2a^2 + 1}{x - a} \\ &= \frac{2x(x - a) - 3ax + 2a^2 + 1}{x - a} \\ &= \frac{2x(x - a) - 3a(x - a) - 3a^2 + 2a^2 + 1}{x - a} \\ &= 2x - 3a + \frac{1 - a^2}{x - a} \end{aligned}
The equations of the asymptotes are x=ax = a and y=2x−3ay = 2x - 3a.

2. Determine the $y$-intercept of the curve.

Substitute x=0x = 0 into the equation of the curve:
y=2(0)2−5a(0)+2a2+10−a=−2a2+1ay = \frac{2(0)^2 - 5a(0) + 2a^2 + 1}{0 - a} = -\frac{2a^2 + 1}{a}
The yy-intercept is (0,−2a2+1a)\left(0, -\frac{2a^2 + 1}{a}\right).

3. Deduce the shape of the branches and outline the sketch.

Since a>1a > 1, we have 1−a2<01 - a^2 < 0.

  • For x<ax < a, x−a<0x - a < 0, which implies 1−a2x−a>0\frac{1 - a^2}{x - a} > 0. The curve lies above the oblique asymptote y=2x−3ay = 2x - 3a and approaches +∞+\infty as x→a−x \to a^-.
  • For x>ax > a, x−a>0x - a > 0, which implies 1−a2x−a<0\frac{1 - a^2}{x - a} < 0. The curve lies below the oblique asymptote and approaches −∞-\infty as x→a+x \to a^+.
    The curve passes through the yy-intercept (0,−2a2+1a)\left(0, -\frac{2a^2 + 1}{a}\right), which is on the left branch.
Answer

Asymptotes:
x=ax = a
y=2x−3ay = 2x - 3a

yy-intercept:
(0,−2a2+1a)\left(0, -\frac{2a^2+1}{a}\right)

[Sketch of curve CC with two branches:

  • For x<ax < a: The curve lies above the dashed line y=2x−3ay = 2x - 3a, passes through (0,−2a2+1a)(0, -\frac{2a^2+1}{a}) on the yy-axis, and approaches the dashed line x=ax = a towards +∞+\infty.
  • For x>ax > a: The curve lies below y=2x−3ay = 2x - 3a and approaches x=ax = a towards −∞-\infty.]

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