The curves C1C_1 and C2C_2 have equations y=2x2+9x2−4y = \frac{2x^2 + 9}{x^2 - 4} and y225−x29=1\frac{y^2}{25} - \frac{x^2}{9} = 1 respectively.

  1. (i)

    Sketch C1C_1 and C2C_2 on the same diagram, including the coordinates of the points where the curves cross the axes, the equations of any asymptotes and the coordinates of the points of intersection of the curves.

  2. (ii)

    Hence, solve the inequality 51+19x2≥2x2+9x2−45\sqrt{1 + \frac{1}{9}x^2} \ge \frac{2x^2 + 9}{x^2 - 4}.

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Worked solution

6(i)

1. Find the asymptotes and axis intercepts of the curve $C_1$.

For C1:y=2x2+9x2−4C_1: y = \frac{2x^2 + 9}{x^2 - 4}

  • Vertical asymptotes: Setting the denominator to zero gives x2−4=0  ⟹  x=−2x^2 - 4 = 0 \implies x = -2 and x=2x = 2.
  • Horizontal asymptote: As x→±∞x \to \pm\infty, y→2x2x2=2  ⟹  y=2y \to \frac{2x^2}{x^2} = 2 \implies y = 2.
  • yy-intercept: When x=0x = 0, y=9−4=−2.25y = \frac{9}{-4} = -2.25. The coordinate is (0,−2.25)(0, -2.25).
  • xx-intercept: Setting y=0y = 0 yields 2x2+9=02x^2 + 9 = 0, which has no real solutions.

2. Find the asymptotes and axis intercepts of the curve $C_2$.

For C2:y225−x29=1C_2: \frac{y^2}{25} - \frac{x^2}{9} = 1

  • This equation represents a standard hyperbola opening along the yy-axis.
  • Asymptotes: Replacing 11 with 00 gives y225−x29=0  ⟹  y=±53x\frac{y^2}{25} - \frac{x^2}{9} = 0 \implies y = \pm \frac{5}{3}x.
  • yy-intercepts: When x=0x = 0, y2=25  ⟹  y=±5y^2 = 25 \implies y = \pm 5. The coordinates are (0,5)(0, 5) and (0,−5)(0, -5).
  • xx-intercepts: Setting y=0y = 0 yields −x29=1-\frac{x^2}{9} = 1, which has no real solutions.

3. Determine the coordinates of the points of intersection between $C_1$ and $C_2$.

To find the intersections, we can use a graphing calculator to solve the system of equations:
y=2x2+9x2−4andy225−x29=1y = \frac{2x^2 + 9}{x^2 - 4} \quad \text{and} \quad \frac{y^2}{25} - \frac{x^2}{9} = 1
Graphing both curves and locating their points of intersection yields the coordinates:
(1.31,−5.46)(1.31, -5.46), (−1.31,−5.46)(-1.31, -5.46), (2.75,6.78)(2.75, 6.78) and (−2.75,6.78)(-2.75, 6.78) (all given to 3 significant figures).

4. Sketch both curves on the same diagram.

(A valid sketch must be drawn on a Cartesian plane including both curves.)

  • Draw the vertical lines x=2x=2, x=−2x=-2 and horizontal line y=2y=2 as dotted lines for the asymptotes of C1C_1.
  • Draw the diagonal dotted lines y=53xy = \frac{5}{3}x and y=−53xy = -\frac{5}{3}x for the asymptotes of C2C_2.
  • Sketch C1C_1: The middle branch is a 'U'-shape opening downwards with a maximum at (0,−2.25)(0, -2.25), bounded by x=±2x = \pm 2. The outer branches lie above y=2y=2 and asymptotically approach x=±2x = \pm 2 and y=2y = 2.
  • Sketch C2C_2: A hyperbola with the upper branch vertex at (0,5)(0, 5) and the lower branch vertex at (0,−5)(0, -5), bounded tightly by its diagonal asymptotes.
  • Ensure the calculated intersection points are correctly positioned relatively on the sketch and labeled.
Answer

C1C_1 features:

  • Asymptotes: x=2x = 2, x=−2x = -2, y=2y = 2
  • Axis intercept: (0,−2.25)(0, -2.25)

C2C_2 features:

  • Asymptotes: y=53xy = \frac{5}{3}x, y=−53xy = -\frac{5}{3}x
  • Axis intercepts: (0,5)(0, 5), (0,−5)(0, -5)

Points of intersection:
(±1.31,−5.46)(\pm 1.31, -5.46) and (±2.75,6.78)(\pm 2.75, 6.78)

(A proper graphical sketch must be drawn on a Cartesian plane showing both C1C_1 and C2C_2 with all the above features clearly labelled.)

6(ii)

1. Relate the given inequality to the graphs of $C_1$ and $C_2$.

From the equation of C2C_2, y225−x29=1  ⟹  y225=1+19x2  ⟹  y=±51+19x2\frac{y^2}{25} - \frac{x^2}{9} = 1 \implies \frac{y^2}{25} = 1 + \frac{1}{9}x^2 \implies y = \pm 5\sqrt{1 + \frac{1}{9}x^2}.
Thus, y=51+19x2y = 5\sqrt{1 + \frac{1}{9}x^2} represents the upper branch of C2C_2 (where y≥0y \ge 0).
The inequality 51+19x2≥2x2+9x2−45\sqrt{1 + \frac{1}{9}x^2} \ge \frac{2x^2 + 9}{x^2 - 4} requires us to find the range of xx for which the upper branch of C2C_2 lies above or intersects C1C_1.

2. Determine the solution using the intersection points and asymptotes from part (i).

From the sketch in part (i), the xx-coordinates of the intersections between the upper branch of C2C_2 and C1C_1 are found (using a graphing calculator) to be x≈−2.75x \approx -2.75 and x≈2.75x \approx 2.75.
The vertical asymptotes of C1C_1 are at x=−2x = -2 and x=2x = 2.
Observing the relative positions of the two curves, the upper branch of C2C_2 is above or meets C1C_1 when x≤−2.75x \le -2.75, in the entire interval between the vertical asymptotes −2<x<2-2 < x < 2, and when x≥2.75x \ge 2.75.

Answer

x≤−2.75 or −2<x<2 or x≥2.75x \le -2.75 \text{ or } -2 < x < 2 \text{ or } x \ge 2.75

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