The curve CC has equation y=3x2+ax+3x+1y = \frac{3x^2 + ax + 3}{x + 1}, where aa is a constant, a≠6a \ne 6.

  1. (a)

    Find the set of values of aa such that CC does not cut the xx-axis.

  2. (b)

    It is given that the equation of CC is y=3x2−6x+3x+1y = \frac{3x^2 - 6x + 3}{x + 1}. Sketch CC and give the equations of any asymptotes. Also, state the coordinates of any points where CC crosses the axes and of any turning points.

  3. (c)

    Let kk be a positive constant. By sketching a suitable graph in the same diagram in part (b), find an inequality satisfied by kk such that the equation

    k2(x−1)2+(3x2−6x+3x+1+12)2=k2k^2(x-1)^2 + \left(\frac{3x^2 - 6x + 3}{x + 1} + 12\right)^2 = k^2

    has 2 real and distinct roots.

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Worked solution

10(a)

1. Form the equation to find the $x$-intercepts of the curve.

At the xx-axis, y=0y = 0.
3x2+ax+3x+1=0\frac{3x^2 + ax + 3}{x + 1} = 0
3x2+ax+3=03x^2 + ax + 3 = 0

2. Apply the condition for the equation to have no real roots.

For the curve not to cut the xx-axis, the quadratic equation must have no real roots. Therefore, its discriminant must be strictly less than zero.
b2−4ac<0b^2 - 4ac < 0
a2−4(3)(3)<0a^2 - 4(3)(3) < 0
a2−36<0a^2 - 36 < 0

3. Solve the quadratic inequality for $a$.

(a−6)(a+6)<0(a - 6)(a + 6) < 0
−6<a<6-6 < a < 6

Answer

−6<a<6-6 < a < 6

10(b)

1. Perform polynomial division to express the equation in a form suitable for finding asymptotes.

y=3x2−6x+3x+1y = \frac{3x^2 - 6x + 3}{x + 1}

Using algebraic long division:
3x2−6x+3=3x(x+1)−9x+3=3x(x+1)−9(x+1)+123x^2 - 6x + 3 = 3x(x + 1) - 9x + 3 = 3x(x + 1) - 9(x + 1) + 12
∴y=3x−9+12x+1\therefore y = 3x - 9 + \frac{12}{x + 1}

2. State the equations of the asymptotes.

From y=3x−9+12x+1y = 3x - 9 + \frac{12}{x + 1}:
As x→−1x \to -1, y→±∞y \to \pm\infty, so the vertical asymptote is x=−1x = -1.
As x→±∞x \to \pm\infty, 12x+1→0\frac{12}{x + 1} \to 0, so the oblique asymptote is y=3x−9y = 3x - 9.

3. Find the coordinates of the intersections with the axes.

yy-intercept: Let x=0x = 0,
y=3(0)2−6(0)+30+1=3  ⟹  (0,3)y = \frac{3(0)^2 - 6(0) + 3}{0 + 1} = 3 \implies (0, 3)

xx-intercept: Let y=0y = 0,
3x2−6x+3x+1=0  ⟹  3(x2−2x+1)=0  ⟹  3(x−1)2=0\frac{3x^2 - 6x + 3}{x + 1} = 0 \implies 3(x^2 - 2x + 1) = 0 \implies 3(x - 1)^2 = 0
x=1  ⟹  (1,0)x = 1 \implies (1, 0)

Intersections with the axes are (0,3)(0, 3) and (1,0)(1, 0).

4. Differentiate the function and equate to zero to find the turning points.

Using y=3x−9+12(x+1)−1y = 3x - 9 + 12(x + 1)^{-1}:
dydx=3−12(x+1)−2=3−12(x+1)2\frac{dy}{dx} = 3 - 12(x + 1)^{-2} = 3 - \frac{12}{(x + 1)^2}

For turning points, dydx=0\frac{dy}{dx} = 0:
3−12(x+1)2=0  ⟹  (x+1)2=43 - \frac{12}{(x + 1)^2} = 0 \implies (x + 1)^2 = 4
x+1=2orx+1=−2x + 1 = 2 \quad \text{or} \quad x + 1 = -2
x=1orx=−3x = 1 \quad \text{or} \quad x = -3

When x=1x = 1, y=3(1)−9+122=0y = 3(1) - 9 + \frac{12}{2} = 0.
When x=−3x = -3, y=3(−3)−9+12−2=−9−9−6=−24y = 3(-3) - 9 + \frac{12}{-2} = -9 - 9 - 6 = -24.

The turning points are (1,0)(1, 0) and (−3,−24)(-3, -24).

5. Sketch the curve $C$ incorporating all identified features.

The sketch must show:

  • A dotted vertical line at x=−1x = -1.
  • A dotted diagonal line for y=3x−9y = 3x - 9.
  • The right branch located above the oblique asymptote and to the right of x=−1x=-1, with a minimum turning point touching the xx-axis at (1,0)(1,0) and crossing the yy-axis at (0,3)(0,3).
  • The left branch located below the oblique asymptote and to the left of x=−1x=-1, with a maximum turning point at (−3,−24)(-3,-24).
Answer

Asymptotes: x=−1x = -1, y=3x−9y = 3x - 9
Intersections with axes: (1,0)(1, 0) and (0,3)(0, 3)
Turning points: (1,0)(1, 0) and (−3,−24)(-3, -24)
(A sketch showing the two branches of the curve approaching the asymptotes and passing through these key points is required)

10(c)

1. Identify the equation of the new graph to sketch.

Substitute y=3x2−6x+3x+1y = \frac{3x^2 - 6x + 3}{x + 1} into the given equation:
k2(x−1)2+(y+12)2=k2k^2(x-1)^2 + (y+12)^2 = k^2
Since kk is a positive constant, dividing through by k2k^2 gives:
(x−1)2+(y+12)2k2=1(x-1)^2 + \frac{(y+12)^2}{k^2} = 1
This represents an ellipse centered at (1,−12)(1, -12) with a horizontal semi-axis of 11 and a vertical semi-axis of kk.

2. Determine the condition for the ellipse to intersect the curve $C$ at exactly two distinct points.

The ellipse only exists for horizontal values bounded by (x−1)2≤1(x-1)^2 \le 1, meaning 0≤x≤20 \le x \le 2. Thus, it will never intersect the left branch of CC (where x<−1x < -1).
From part (b), for x>−1x > -1, curve CC has a minimum turning point at (1,0)(1, 0).
At the boundaries x=0x = 0 and x=2x = 2, the ellipse points are strictly at y=−12y = -12, which are below curve CC (where y=3y = 3 and y=1y = 1 respectively).
The highest point of the ellipse occurs at x=1x = 1, yielding y=−12+ky = -12 + k.
For the equation to have exactly 2 real and distinct roots, the upper half of the ellipse must intersect curve CC at exactly 2 points in the interval (0,2)(0, 2).
Since CC is concave up and the upper half of the ellipse is concave down, they will intersect exactly twice if and only if the peak of the ellipse is strictly above the minimum of CC:
−12+k>0-12 + k > 0
k>12k > 12

Answer

k>12k > 12

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