The hyperbola C1C_1 has equation (x−2)232−y222=1\frac{(x-2)^2}{3^2} - \frac{y^2}{2^2} = 1.

  1. (a)

    Sketch C1C_1, labelling the equations of the asymptotes and coordinates of the axial intercepts.

  2. (b)

    The curve C2C_2 has equation (x−2)2+y2=k2(x-2)^2 + y^2 = k^2, where k>0k > 0.

    State the value of kk such that C1C_1 and C2C_2 intersect exactly twice.

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Worked solution

1(a)

1. Find the axial intercepts of the hyperbola.

To find the xx-intercepts, set y=0y = 0:
(x−2)232−0=1\frac{(x-2)^2}{3^2} - 0 = 1
(x−2)2=9(x-2)^2 = 9
x−2=±3x - 2 = \pm 3
x=5orx=−1x = 5 \quad \text{or} \quad x = -1
The xx-intercepts are (5,0)(5, 0) and (−1,0)(-1, 0).

To find the yy-intercepts, set x=0x = 0:
(−2)29−y24=1\frac{(-2)^2}{9} - \frac{y^2}{4} = 1
49−y24=1\frac{4}{9} - \frac{y^2}{4} = 1
−y24=59-\frac{y^2}{4} = \frac{5}{9}
Since y2≥0y^2 \ge 0, there are no real solutions. Hence, there are no yy-intercepts.

2. Determine the equations of the asymptotes.

The hyperbola is in the standard form (x−h)2a2−(y−k)2b2=1\frac{(x-h)^2}{a^2} - \frac{(y-k)^2}{b^2} = 1 with center (h,k)=(2,0)(h, k) = (2, 0), a=3a = 3, and b=2b = 2.
The equations of the asymptotes are given by y−k=±ba(x−h)y - k = \pm \frac{b}{a}(x - h).
Substituting the values:
y−0=±23(x−2)y - 0 = \pm \frac{2}{3}(x - 2)
So the asymptotes are:
y=23(x−2)andy=−23(x−2)y = \frac{2}{3}(x - 2) \quad \text{and} \quad y = -\frac{2}{3}(x - 2)

3. Sketch the graph incorporating all the key features.

  1. Draw a set of coordinate axes and mark the center at (2,0)(2, 0).
  2. Draw the dashed asymptotic lines y=23(x−2)y = \frac{2}{3}(x - 2) and y=−23(x−2)y = -\frac{2}{3}(x - 2) passing through the center (2,0)(2,0).
  3. Mark the vertices (which are also the xx-intercepts) at (−1,0)(-1, 0) and (5,0)(5, 0).
  4. Sketch the two branches of the hyperbola opening horizontally from the vertices, approaching the asymptotes as x→±∞x \to \pm\infty.
Answer

A sketched hyperbola consisting of two branches opening horizontally.

Axial intercepts: (−1,0)(-1, 0) and (5,0)(5, 0)
Asymptotes: y=23(x−2)y = \frac{2}{3}(x - 2) and y=−23(x−2)y = -\frac{2}{3}(x - 2)

1(b)

1. Use the geometric properties of the given hyperbola and circle to deduce the value of k for exactly two intersections.

The hyperbola C1C_1 and the circle C2C_2 both share the same centre at (2,0)(2, 0).
From the equation of C1C_1, the horizontal distance from the centre to its vertices is a=3a = 3.
For the circle C2C_2 with radius kk to intersect the hyperbola exactly twice, it must be tangent to the two branches of the hyperbola at its vertices.
Therefore, the radius of the circle must exactly equal the distance from the centre to the vertices.
k=3k = 3

Answer

k=3k = 3

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