Sketch the graph of (x−2)2+4(y+2)2=16(x-2)^2 + 4(y+2)^2 = 16, stating the coordinates of all the vertices. On the same diagram, sketch the graph of y=x2−2x+6x+1y = \frac{x^2 - 2x + 6}{x + 1}, stating the equations of any asymptotes, the coordinates of turning points and the points of intersection with the axes. Find the range of values of mm, where m>0m > 0, such that (x−2)2+4m2(x2−2x+6x+1+2)2=16(x-2)^2 + 4m^2 \left( \frac{x^2 - 2x + 6}{x + 1} + 2 \right)^2 = 16 has no real solutions.

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Worked solution

7(i)

1. Rewrite the given equation into the standard form of an ellipse.

(x−2)2+4(y+2)2=16(x-2)^2 + 4(y+2)^2 = 16
Divide by 16 throughout:
(x−2)216+(y+2)24=1\frac{(x-2)^2}{16} + \frac{(y+2)^2}{4} = 1

2. Identify the centre, major, and minor axes lengths to determine the vertices.

Comparing with (x−h)2a2+(y−k)2b2=1\frac{(x-h)^2}{a^2} + \frac{(y-k)^2}{b^2} = 1:
Centre (h,k)=(2,−2)(h, k) = (2, -2)
Horizontal semi-axis a=16=4a = \sqrt{16} = 4
Vertical semi-axis b=4=2b = \sqrt{4} = 2

Horizontal vertices are at (2±4,−2)(2 \pm 4, -2):
⇒(−2,−2)\Rightarrow (-2, -2) and (6,−2)(6, -2)

Vertical vertices are at (2,−2±2)(2, -2 \pm 2):
⇒(2,0)\Rightarrow (2, 0) and (2,−4)(2, -4)

3. Sketch the ellipse indicating the calculated vertices.

Draw a smooth oval shape centered at (2,−2)(2, -2). Ensure the curve passes through the points (−2,−2)(-2, -2), (6,−2)(6, -2), (2,0)(2, 0), and (2,−4)(2, -4). The graph should touch the xx-axis at (2,0)(2, 0) and not cross it.

Answer

Vertices: (−2,−2)(-2, -2), (6,−2)(6, -2), (2,0)(2, 0), (2,−4)(2, -4)
(A sketch of an ellipse centered at (2,−2)(2,-2) passing through these points is required)

7(ii)

1. Rewrite the rational function using algebraic division to identify the asymptotes.

y = rac{x^2 - 2x + 6}{x + 1} = rac{x(x + 1) - 3(x + 1) + 9}{x + 1} = x - 3 + rac{9}{x + 1}
The vertical asymptote is x=−1x = -1.
As x→±∞x \to \pm \infty, 9x+1→0\frac{9}{x + 1} \to 0, so the oblique asymptote is y=x−3y = x - 3.

2. Find the coordinates of the turning points using differentiation.

dydx=1−9(x+1)2\frac{\mathrm{d}y}{\mathrm{d}x} = 1 - \frac{9}{(x + 1)^2}
Set dydx=0\frac{\mathrm{d}y}{\mathrm{d}x} = 0 for turning points:
1−9(x+1)2=0  ⟹  (x+1)2=9  ⟹  x+1=±31 - \frac{9}{(x + 1)^2} = 0 \implies (x + 1)^2 = 9 \implies x + 1 = \pm 3
x=2orx=−4x = 2 \quad \text{or} \quad x = -4
When x=2x = 2, y=2−3+93=2y = 2 - 3 + \frac{9}{3} = 2.
When x=−4x = -4, y=−4−3+9−3=−10y = -4 - 3 + \frac{9}{-3} = -10.
The turning points are (2,2)(2, 2) and (−4,−10)(-4, -10).

3. Determine the points of intersection with the axes.

For the yy-intercept, set x=0x = 0:
y=0−0+60+1=6y = \frac{0 - 0 + 6}{0 + 1} = 6
For the xx-intercepts, set y=0y = 0:
x2−2x+6=0x^2 - 2x + 6 = 0
Discriminant Δ=(−2)2−4(1)(6)=−20<0\Delta = (-2)^2 - 4(1)(6) = -20 < 0.
Since the discriminant is negative, there are no real roots, meaning the curve does not intersect the xx-axis.
The point of intersection with the axes is (0,6)(0, 6).

4. Sketch the graph of the rational function on the same diagram.

Draw the two branches of the rational curve on the same axes used in part (i). Ensure the branches approach the asymptotes x=−1x = -1 and y=x−3y = x - 3, passing through (0,6)(0, 6), with a local minimum at (2,2)(2, 2) and a local maximum at (−4,−10)(-4, -10).

Answer

Asymptotes: x=−1x = -1, y=x−3y = x - 3
Turning points: (2,2)(2, 2) and (−4,−10)(-4, -10)
Intersection with axes: (0,6)(0, 6)
(A sketch of the rational curve indicating these key features must be drawn on the diagram from part (i).)

7(iii)

1. Relate the given equation to algebraic bounds.

Let y=x2−2x+6x+1y = \frac{x^2 - 2x + 6}{x + 1}. The given equation is:
(x−2)2+4m2(y+2)2=16(x-2)^2 + 4m^2(y+2)^2 = 16
For any real solution xx, we know (x−2)2≥0(x-2)^2 \ge 0, which implies:
4m2(y+2)2≤164m^2(y+2)^2 \le 16
m2(y+2)2≤4m^2(y+2)^2 \le 4
Since m>0m > 0, taking the square root gives:
m∣y+2∣≤2  ⟹  ∣y+2∣≤2mm|y+2| \le 2 \implies |y+2| \le \frac{2}{m}

2. Use the range of $y$ from part (ii) to find the condition for no real solutions.

From part (ii), the turning points of yy are (2,2)(2, 2) and (−4,−10)(-4, -10). The range of values for yy is y≥2y \ge 2 or y≤−10y \le -10.
Evaluating ∣y+2∣|y+2| for these ranges:
For y≥2y \ge 2, y+2≥4  ⟹  ∣y+2∣≥4y + 2 \ge 4 \implies |y+2| \ge 4.
For y≤−10y \le -10, y+2≤−8  ⟹  ∣y+2∣≥8y + 2 \le -8 \implies |y+2| \ge 8.
Thus, the minimum value of ∣y+2∣|y+2| for any valid xx is 44.
For the equation to have no real solutions, the necessary values of ∣y+2∣|y+2| must lie strictly outside the possible region ∣y+2∣≤2m|y+2| \le \frac{2}{m}.
4>2m4 > \frac{2}{m}
Since m>0m > 0,
4m>2  ⟹  m>124m > 2 \implies m > \frac{1}{2}

Answer

m>12m > \frac{1}{2}

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