Sketch the graph of (y+2)2=4(x−1)2−9(y+2)^2 = 4(x-1)^2 - 9, showing its main relevant features. Hence find the range of values of kk such that there are no real roots to the equation 9+(y+2)2=4(k2−1)2.9 + (y+2)^2 = 4(k^2 - 1)^2. The diagram below shows the graph of y=h(3−x)y = h(3 - x). On separate diagrams, sketch the graphs of Sketch the graph of y=h(x+3)y = h(x+3). Sketch the graph of y=12h(x)y = \tfrac{1}{2}h(x).

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Worked solution

9(a)(i)

1. Rearrange the equation into the standard form of a hyperbola.

(y+2)2=4(x−1)2−9(y+2)^2 = 4(x-1)^2 - 9
4(x−1)2−(y+2)2=94(x-1)^2 - (y+2)^2 = 9
(x−1)294−(y+2)29=1\frac{(x-1)^2}{\frac{9}{4}} - \frac{(y+2)^2}{9} = 1
This represents a hyperbola with a horizontal transverse axis and center (1,−2)(1, -2).

2. Determine the coordinates of the vertices.

The vertices lie on the horizontal line y=−2y = -2 passing through the center:
(x−1)294=1  ⟹  x−1=±32\frac{(x-1)^2}{\frac{9}{4}} = 1 \implies x - 1 = \pm \frac{3}{2}
x=2.5 or x=−0.5x = 2.5 \text{ or } x = -0.5
The vertices are at (2.5,−2)(2.5, -2) and (−0.5,−2)(-0.5, -2).

3. Find the equations of the asymptotes.

Set the right-hand side of the standard equation to 0:
(x−1)294−(y+2)29=0\frac{(x-1)^2}{\frac{9}{4}} - \frac{(y+2)^2}{9} = 0
(y+2)2=4(x−1)2  ⟹  y+2=±2(x−1)(y+2)^2 = 4(x-1)^2 \implies y+2 = \pm 2(x-1)
For y+2=2(x−1)y+2 = 2(x-1), we get the asymptote y=2x−4y = 2x - 4.
For y+2=−2(x−1)y+2 = -2(x-1), we get the asymptote y=−2xy = -2x (which passes through the origin).

4. Calculate the axial intercepts.

For xx-intercepts, set y=0y = 0:
(0+2)2=4(x−1)2−9  ⟹  4(x−1)2=13(0+2)^2 = 4(x-1)^2 - 9 \implies 4(x-1)^2 = 13
x−1=±132  ⟹  x=1±132x - 1 = \pm\frac{\sqrt{13}}{2} \implies x = 1 \pm \frac{\sqrt{13}}{2}
The xx-intercepts are (1−132,0)\left(1 - \frac{\sqrt{13}}{2}, 0\right) and (1+132,0)\left(1 + \frac{\sqrt{13}}{2}, 0\right).

For yy-intercepts, set x=0x = 0:
(y+2)2=4(0−1)2−9=−5(y+2)^2 = 4(0-1)^2 - 9 = -5
Since there are no real solutions, the graph does not cross the yy-axis.

Answer

A sketch of a two-branch hyperbola opening left and right, with:

  • Vertices at (−0.5,−2)(-0.5, -2) and (2.5,−2)(2.5, -2)
  • Asymptotes y=2x−4y = 2x - 4 and y=−2xy = -2x (the latter passing exactly through the origin)
  • xx-intercepts at (1−132,0)\left(1 - \frac{\sqrt{13}}{2}, 0\right) and (1+132,0)\left(1 + \frac{\sqrt{13}}{2}, 0\right)
  • No yy-intercepts.

9(a)(ii)

1. Relate the given equation to the sketched graph to establish the domain condition for no real roots.

Rearranging the given equation:
(y+2)2=4(k2−1)2−9(y+2)^2 = 4(k^2 - 1)^2 - 9
Comparing this to the equation of the graph from (a)(i), (y+2)2=4(x−1)2−9(y+2)^2 = 4(x-1)^2 - 9, we can see it is equivalent to substituting x=k2x = k^2.
From the graph's features, the hyperbola intersects its central horizontal axis (y=−2y=-2) when 4(x−1)2−9=0  ⟹  x=−0.5 or x=2.54(x-1)^2 - 9 = 0 \implies x = -0.5 \text{ or } x = 2.5. The curve only exists for x≤−0.5x \le -0.5 and x≥2.5x \ge 2.5.
For the equation to have no real roots for yy, the line x=k2x = k^2 must fall in the gap where the graph does not exist:
−0.5<x<2.5-0.5 < x < 2.5
Substituting x=k2x = k^2:
−0.5<k2<2.5-0.5 < k^2 < 2.5

2. Solve the inequality to find the range of values for $k$.

Since kk is a real number, k2≥0k^2 \ge 0, which means −0.5<k2-0.5 < k^2 is automatically satisfied.
We only need to solve the upper bound:
k2<2.5k^2 < 2.5
k2<52k^2 < \frac{5}{2}
−52<k<52-\sqrt{\frac{5}{2}} < k < \sqrt{\frac{5}{2}}
−102<k<102-\frac{\sqrt{10}}{2} < k < \frac{\sqrt{10}}{2}

Answer

−102<k<102-\frac{\sqrt{10}}{2} < k < \frac{\sqrt{10}}{2}

9(b)(i)

1. Relate the target function to the given function.

Let the given function be f(x)=h(3−x)f(x) = h(3 - x).
We want to sketch y=h(x+3)y = h(x + 3).
Substituting −x-x for xx in f(x)f(x) gives:
f(−x)=h(3−(−x))=h(3+x)f(-x) = h(3 - (-x)) = h(3 + x)
Thus, the required graph of y=h(x+3)y = h(x + 3) is simply the reflection of the original graph y=f(x)y = f(x) in the yy-axis.

2. Determine the new positions of the key features after reflection.

Applying the transformation x↦−xx \mapsto -x to all key features:

  • Vertical asymptote x=2  ⟹  x=−2x = 2 \implies x = -2.
  • Vertical asymptote x=0  ⟹  x=0x = 0 \implies x = 0 (unchanged).
  • The xx-axis asymptote as x→∞  ⟹  x \to \infty \implies asymptote as x→−∞x \to -\infty.
  • Given point (−3,−2)  ⟹  (3,−2)(-3, -2) \implies (3, -2).
  • Turning point (1,−4)  ⟹  (−1,−4)(1, -4) \implies (-1, -4).

3. Describe the final sketched graph.

Sketch the reflected branches on the Cartesian plane:

  • For x<−2x < -2, the curve lies above the xx-axis, approaching y=0y = 0 as x→−∞x \to -\infty and x=−2x = -2 as a vertical asymptote.
  • For −2<x<0-2 < x < 0, the curve is a downward loop with a turning point at (−1,−4)(-1, -4), bound by asymptotes x=−2x = -2 and x=0x = 0.
  • For x>0x > 0, the curve passes through (3,−2)(3, -2), crosses the xx-axis, and goes to +∞+\infty as it approaches the yy-axis.
Answer

A sketch showing a reflection of the original graph in the yy-axis:

  • Vertical asymptotes at x=−2x = -2 and x=0x = 0.
  • Left branch (x<−2x < -2): above the xx-axis, approaching y=0y=0 as x→−∞x \to -\infty.
  • Middle branch (−2<x<0-2 < x < 0): downward 'U'-shape below the xx-axis with turning point at (−1,−4)(-1, -4).
  • Right branch (x>0x > 0): passing through (3,−2)(3, -2) and crossing the xx-axis to approach +∞+\infty as x→0+x \to 0^+.

9(b)(ii)

1. Identify the relationship mapping the coordinates of the given graph $y = h(3-x)$ to the required graph $y = \frac{1}{2}h(x)$.

Let the given graph be represented by the coordinates (x0,y0)(x_0, y_0), where y0=h(3−x0)y_0 = h(3-x_0).
We want to plot the points (x,y)(x, y) such that y=12h(x)y = \frac{1}{2}h(x).
Equating the arguments of the function hh, we get x=3−x0x = 3 - x_0.
Equating the outputs gives 2y=h(x)=h(3−x0)=y0  ⟹  y=12y02y = h(x) = h(3-x_0) = y_0 \implies y = \frac{1}{2} y_0.
Thus, every point (x0,y0)(x_0, y_0) on the original graph maps to (3−x0,12y0)(3 - x_0, \frac{1}{2}y_0) on the new graph.
(Geometrically, this corresponds to a reflection in the line x=1.5x = 1.5, followed by a vertical scaling by a factor of 12\frac{1}{2}.)

2. Apply the transformation to the given key points.

Using the coordinate mapping (x0,y0)↦(3−x0,12y0)(x_0, y_0) \mapsto (3 - x_0, \frac{1}{2}y_0):

  • The turning point (1,−4)(1, -4) maps to (3−1,12(−4))=(2,−2)(3 - 1, \frac{1}{2}(-4)) = (2, -2).
  • The given reference point (−3,−2)(-3, -2) maps to (3−(−3),12(−2))=(6,−1)(3 - (-3), \frac{1}{2}(-2)) = (6, -1).
  • The xx-intercept marked at x0=−1x_0 = -1, which is (−1,0)(-1, 0), maps to (3−(−1),12(0))=(4,0)(3 - (-1), \frac{1}{2}(0)) = (4, 0).

3. Apply the transformation to the asymptotes and describe the resulting sketch.

Using the same mapping for the vertical asymptotes (x0=0x_0 = 0 and x0=2x_0 = 2):

  • x0=0↦x=3−0=3x_0 = 0 \mapsto x = 3 - 0 = 3
  • x0=2↦x=3−2=1x_0 = 2 \mapsto x = 3 - 2 = 1

The horizontal asymptote y0=0y_0 = 0 maps to y=12(0)=0y = \frac{1}{2}(0) = 0.

Reversing the horizontal orientation (due to the −x0-x_0 transformation) changes the left-to-right order of the branches:

  • The original right branch (x0>2x_0 > 2) becomes the new left branch (x<1x < 1).
  • The original middle loop (0<x0<20 < x_0 < 2) becomes the new middle loop (1<x<31 < x < 3) with its peak turning point mapped to (2,−2)(2, -2).
  • The original left branch (x0<0x_0 < 0) becomes the new right branch (x>3x > 3), decreasing from the asymptote x=3x=3, crossing (4,0)(4, 0), and passing through (6,−1)(6, -1).
Answer

A sketch of the graph of y=12h(x)y = \frac{1}{2}h(x) showing:

  • Vertical asymptotes at x=1x = 1 and x=3x = 3.
  • Horizontal asymptote y=0y = 0 (xx-axis).
  • A curve for x<1x < 1 entirely above the xx-axis, approaching y=0y=0 as x→−∞x \to -\infty and going to +∞+\infty as x→1−x \to 1^-.
  • An inverted U-shape branch for 1<x<31 < x < 3 entirely below the xx-axis, with a local maximum turning point at (2,−2)(2, -2).
  • A curve for x>3x > 3 starting from +∞+\infty near x=3+x=3^+, crossing the xx-axis at (4,0)(4, 0), passing through (6,−1)(6, -1), and approaching y=0y=0 as x→∞x \to \infty.

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