The curve CC has the equation y=ax2−3x+bx−cy = \dfrac{ax^{2} - 3x + b}{x - c} where aa, bb, and cc are real constants. It is given that the equations of asymptotes of CC are x=2x = 2 and y=x−1y = x - 1. Also, CC passes through the point P(0,−32)P\left(0, -\frac{3}{2}\right).

  1. (i)

    Explain why the value of aa is 1. State the value of cc and show that b=3b = 3.

  2. (ii)

    Sketch the curve CC, indicating clearly the equations of asymptotes, axial intercepts and stationary points.

  3. (iii)

    Find the exact equation of the normal to CC at the point PP.

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Worked solution

4(i)

1. Explain why $a = 1$ using the oblique asymptote.

The rational function is y=ax2−3x+bx−cy = \dfrac{ax^2 - 3x + b}{x - c}.
Performing polynomial long division, the quotient begins with the term ax2x=ax\dfrac{ax^2}{x} = ax.
Since the oblique asymptote is given as y=x−1y = x - 1, the linear quotient must be x−1x - 1.
Comparing the coefficient of xx, we deduce that a=1a = 1.

2. State the value of $c$ by identifying the vertical asymptote.

The vertical asymptote occurs where the denominator is zero, so x−c=0  ⟹  x=cx - c = 0 \implies x = c.
Given that the vertical asymptote is x=2x = 2, we have:
c=2c = 2

3. Show that $b = 3$ by substituting the given point $P$.

The curve passes through the point P(0,−32)P\left(0, -\frac{3}{2}\right).
Substituting x=0x = 0, y=−32y = -\frac{3}{2}, and c=2c = 2 into the curve equation y=x2−3x+bx−cy = \dfrac{x^2 - 3x + b}{x - c}:
−32=0−0+b0−2-\frac{3}{2} = \dfrac{0 - 0 + b}{0 - 2}
−32=b−2-\frac{3}{2} = \dfrac{b}{-2}
b=(−32)(−2)=3 (shown)b = \left(-\frac{3}{2}\right)(-2) = 3 \text{ (shown)}

Answer

c=2c = 2 and b=3b = 3 (shown)

4(ii)

1. Determine the axial intercepts of the curve.

From part (i), a=1a = 1, b=3b = 3, and c=2c = 2, so the equation of the curve CC is:
y=x2−3x+3x−2y = \frac{x^2 - 3x + 3}{x - 2}
To find the yy-intercept, let x=0x = 0:
y=3−2=−1.5y = \frac{3}{-2} = -1.5
The point (0,−1.5)(0, -1.5) is the yy-intercept (also given as point PP).

To find any xx-intercepts, let y=0y = 0:
x2−3x+3=0x^2 - 3x + 3 = 0
Checking the discriminant, Δ=(−3)2−4(1)(3)=9−12=−3\Delta = (-3)^2 - 4(1)(3) = 9 - 12 = -3.
Since Δ<0\Delta < 0, there are no real roots, meaning the curve has no xx-intercepts.

2. Find the coordinates of the stationary points.

Using the asymptotes y=x−1y = x - 1 and x=2x = 2, or by algebraic division, CC can be rewritten as:
y=x−1+1x−2y = x - 1 + \frac{1}{x - 2}
Differentiating with respect to xx:
dydx=1−1(x−2)2\frac{dy}{dx} = 1 - \frac{1}{(x - 2)^2}
At stationary points, dydx=0\frac{dy}{dx} = 0:
1−1(x−2)2=01 - \frac{1}{(x - 2)^2} = 0
(x−2)2=1(x - 2)^2 = 1
x−2=1orx−2=−1x - 2 = 1 \quad \text{or} \quad x - 2 = -1
x=3orx=1x = 3 \quad \text{or} \quad x = 1
Substitute the xx-values back into the equation of the curve to find the corresponding yy-coordinates:
When x=3x = 3, y=3−1+11=3y = 3 - 1 + \frac{1}{1} = 3.
When x=1x = 1, y=1−1+1−1=−1y = 1 - 1 + \frac{1}{-1} = -1.
Therefore, the stationary points are (1,−1)(1, -1) and (3,3)(3, 3).

3. Sketch the curve with all indicated key features.

Draw the coordinate axes.

  • Draw and label the vertical asymptote x=2x = 2 and oblique asymptote y=x−1y = x - 1 with dashed lines.
  • Mark the yy-intercept at (0,−1.5)(0, -1.5).
  • Plot and label the local maximum turning point at (1,−1)(1, -1) and the local minimum turning point at (3,3)(3, 3).
  • Sketch the two branches of the curve such that they pass through the respective points and asymptotically approach the dashed lines.
Answer

A valid sketch must clearly show:

  • Two branches of the hyperbola.
  • Asymptotes drawn as dashed lines and labelled as x=2x = 2 and y=x−1y = x - 1.
  • The yy-intercept plotted and labelled at (0,−1.5)(0, -1.5).
  • Two stationary points plotted and labelled at (1,−1)(1, -1) and (3,3)(3, 3).

4(iii)

1. Find the derivative $\frac{dy}{dx}$ of the curve.

From part (i), a=1a=1, b=3b=3 and c=2c=2, so the equation of the curve is:
y=x2−3x+3x−2y = \frac{x^2 - 3x + 3}{x - 2}
Using the quotient rule:
dydx=(x−2)(2x−3)−(x2−3x+3)(1)(x−2)2\frac{dy}{dx} = \frac{(x - 2)(2x - 3) - (x^2 - 3x + 3)(1)}{(x - 2)^2}

2. Calculate the gradient of the tangent at $P\left(0, -\frac{3}{2}\right)$.

Substitute x=0x = 0 into dydx\frac{dy}{dx}:
dydx∣x=0=(−2)(−3)−(3)(1)(−2)2=6−34=34\left.\frac{dy}{dx}\right|_{x=0} = \frac{(-2)(-3) - (3)(1)}{(-2)^2} = \frac{6 - 3}{4} = \frac{3}{4}

3. Find the gradient of the normal and the exact equation of the normal.

The gradient of the normal mnm_n is the negative reciprocal of the tangent gradient:
mn=−13/4=−43m_n = -\frac{1}{3/4} = -\frac{4}{3}
Using the point-gradient form for P(0,−32)P\left(0, -\frac{3}{2}\right):
y−(−32)=−43(x−0)y - \left(-\frac{3}{2}\right) = -\frac{4}{3}(x - 0)
y=−43x−32y = -\frac{4}{3}x - \frac{3}{2}

Answer

y=−43x−32y = -\frac{4}{3}x - \frac{3}{2}

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