The curve CC has equation y=x2+ax+bx+1y = \frac{x^2 + ax + b}{x + 1}, where aa and bb are constants. It is given that CC has a minimum point at (2,0)(2, 0).

  1. (i)

    Show that a=−4a = -4 and b=4b = 4.

  2. (ii)

    Sketch the graph of CC, indicating clearly the equations of any asymptotes and the coordinates of any turning points and axial intercepts.

  3. (iii)

    By adding an appropriate graph to your sketch in (ii), deduce the number of real roots of the equation

    x3+ax2+(b+1)x+1=0.x^3 + ax^2 + (b + 1)x + 1 = 0.

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Worked solution

5(i)

1. Form an equation in $a$ and $b$ using the given point on the curve.

Since the curve passes through the minimum point (2,0)(2, 0), we substitute x=2x = 2 and y=0y = 0 into y=x2+ax+bx+1y = \frac{x^2 + ax + b}{x + 1}:
0=22+2a+b2+10 = \frac{2^2 + 2a + b}{2 + 1}
0=4+2a+b30 = \frac{4 + 2a + b}{3}
4+2a+b=04 + 2a + b = 0
2a+b=−4— (1)2a + b = -4 \quad \text{--- (1)}

2. Find $\frac{dy}{dx}$ using the quotient rule.

dydx=(x+1)ddx(x2+ax+b)−(x2+ax+b)ddx(x+1)(x+1)2\frac{dy}{dx} = \frac{(x + 1)\frac{d}{dx}(x^2 + ax + b) - (x^2 + ax + b)\frac{d}{dx}(x + 1)}{(x + 1)^2}
dydx=(x+1)(2x+a)−(x2+ax+b)(1)(x+1)2\frac{dy}{dx} = \frac{(x + 1)(2x + a) - (x^2 + ax + b)(1)}{(x + 1)^2}
dydx=2x2+ax+2x+a−x2−ax−b(x+1)2\frac{dy}{dx} = \frac{2x^2 + ax + 2x + a - x^2 - ax - b}{(x + 1)^2}
dydx=x2+2x+a−b(x+1)2\frac{dy}{dx} = \frac{x^2 + 2x + a - b}{(x + 1)^2}

3. Use the condition for a stationary point to form a second equation.

At the minimum point, the derivative is zero, so dydx=0\frac{dy}{dx} = 0. Substituting x=2x = 2:
22+2(2)+a−b(2+1)2=0\frac{2^2 + 2(2) + a - b}{(2 + 1)^2} = 0
8+a−b9=0\frac{8 + a - b}{9} = 0
8+a−b=08 + a - b = 0
a−b=−8— (2)a - b = -8 \quad \text{--- (2)}

4. Solve the simultaneous equations to find $a$ and $b$.

Adding equation (1) and equation (2):
(2a+b)+(a−b)=−4+(−8)(2a + b) + (a - b) = -4 + (-8)
3a=−123a = -12
a=−4a = -4

Substitute a=−4a = -4 into equation (1):
2(−4)+b=−42(-4) + b = -4
−8+b=−4-8 + b = -4
b=4b = 4

Thus, a=−4a = -4 and b=4b = 4 (shown).

Answer

a=−4,b=4a = -4, b = 4 (shown)

5(ii)

1. Substitute the given values of $a$ and $b$ to find the equation of $C$ and its axial intercepts.

Substitute a=−4a = -4 and b=4b = 4 from part (i):
y=x2−4x+4x+1=(x−2)2x+1y = \frac{x^2 - 4x + 4}{x + 1} = \frac{(x-2)^2}{x+1}
For the yy-intercept, let x=0x = 0:
y=41=4y = \frac{4}{1} = 4
For the xx-intercept, let y=0y = 0:
(x−2)2=0  ⟹  x=2(x-2)^2 = 0 \implies x = 2
The axial intercepts are (0,4)(0, 4) and (2,0)(2, 0).

2. Find the equations of the vertical and oblique asymptotes.

By algebraic long division (or algebraic manipulation):
y=x(x+1)−5x+4x+1=x(x+1)−5(x+1)+9x+1=x−5+9x+1y = \frac{x(x+1) - 5x + 4}{x+1} = \frac{x(x+1) - 5(x+1) + 9}{x+1} = x - 5 + \frac{9}{x + 1}
The vertical asymptote occurs where the denominator is zero:
x+1=0  ⟹  x=−1x + 1 = 0 \implies x = -1
As x→±∞x \to \pm \infty, 9x+1→0\frac{9}{x+1} \to 0. Therefore, the oblique asymptote is:
y=x−5y = x - 5

3. Determine the coordinates of the turning points analytically or via a graphing calculator.

Differentiating yy with respect to xx:
dydx=1−9(x+1)2\frac{dy}{dx} = 1 - \frac{9}{(x + 1)^2}
At the turning points, dydx=0\frac{dy}{dx} = 0:
1−9(x+1)2=0  ⟹  (x+1)2=91 - \frac{9}{(x + 1)^2} = 0 \implies (x + 1)^2 = 9
x+1=±3  ⟹  x=2orx=−4x + 1 = \pm 3 \implies x = 2 \quad \text{or} \quad x = -4
When x=2x = 2, y=0y = 0 (this confirms the given minimum point).
When x=−4x = -4, y=−4−5+9−4+1=−9−3=−12y = -4 - 5 + \frac{9}{-4 + 1} = -9 - 3 = -12.
The turning points are the minimum at (2,0)(2, 0) and the maximum at (−4,−12)(-4, -12).

4. Sketch the graph of $C$ incorporating all identified features.

Draw the axes and the asymptotes x=−1x = -1 and y=x−5y = x - 5.
Sketch the two branches of the rational curve:

  • The upper right branch approaches x=−1x = -1 as x→−1+x \to -1^+ and y=x−5y = x - 5 as x→∞x \to \infty, passing through (0,4)(0,4) and turning at the local minimum (2,0)(2,0).
  • The lower left branch approaches x=−1x = -1 as x→−1−x \to -1^- and y=x−5y = x - 5 as x→−∞x \to -\infty, turning at the local maximum (−4,−12)(-4,-12).
Answer

Graph Sketch Requirements:

  • Curve: Two distinct continuous branches appropriate for a rational function.
  • Asymptotes: Vertical line x=−1x = -1 and oblique line y=x−5y = x - 5 (drawn as dotted lines and labelled).
  • Turning Points: Maximum point clearly indicated at (−4,−12)(-4, -12) and minimum point at (2,0)(2, 0).
  • Axial Intercepts: Curve cuts the yy-axis at (0,4)(0, 4) and touches the xx-axis at (2,0)(2, 0).

5(iii)

1. Rearrange the given cubic equation to express it in terms of the function for curve $C$.

Substitute a=−4a = -4 and b=4b = 4 into the given equation:
x3−4x2+5x+1=0x^3 - 4x^2 + 5x + 1 = 0
Rearranging to isolate terms that match the numerator of CC:
x3−4x2+4x+x+1=0x^3 - 4x^2 + 4x + x + 1 = 0
x(x2−4x+4)=−(x+1)x(x^2 - 4x + 4) = -(x + 1)
Dividing both sides by x(x+1)x(x + 1) (verifying that x=0x = 0 and x=−1x = -1 are not roots):
x2−4x+4x+1=−1x\frac{x^2 - 4x + 4}{x + 1} = -\frac{1}{x}
The appropriate graph to add is y=−1xy = -\frac{1}{x}.

2. Deduce the number of real roots by analyzing the intersections of the two graphs.

Imagine or sketch y=−1xy = -\frac{1}{x} on the same axes as CC:

  • For x>0x > 0, the graph of CC is strictly non-negative (y≥0y \ge 0) while y=−1x<0y = -\frac{1}{x} < 0, meaning no intersection.
  • For x<−1x < -1, the graph of CC is entirely below the xx-axis (its local maximum is at y=−12y = -12) while y=−1x>0y = -\frac{1}{x} > 0, meaning no intersection.
  • For −1<x<0-1 < x < 0, as x→−1+x \to -1^+, C(x)→∞C(x) \to \infty and y=−1x→1y = -\frac{1}{x} \to 1. As x→0−x \to 0^-, C(x)→4C(x) \to 4 and y=−1x→∞y = -\frac{1}{x} \to \infty.

Since C(x)C(x) is continuous and monotonic decreasing on this interval while y=−1xy = -\frac{1}{x} is continuous and monotonic increasing, the two curves cross exactly once in (−1,0)(-1, 0).
Therefore, there is 11 point of intersection, meaning the original equation has 11 real root.

Answer

1

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