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Exercise 10.4 · Q8

Q.Find the equation of the hyperbola satisfying the given conditions: Vertices (0,±5)(0, \pm 5), foci (0,±8)(0, \pm 8).

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The hyperbola is vertical (transverse axis along the y-axis) because the vertices and foci have the same x-coordinate. Using the standard form y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1, we have a=5a = 5 and c=8c = 8, so b2=c2−a2=64−25=39b^2 = c^2 - a^2 = 64 - 25 = 39. The equation is y225−x239=1\frac{y^2}{25} - \frac{x^2}{39} = 1.

The first thing to notice is the coordinates of the vertices and foci. Both are given as (0,±5)(0, \pm 5) and (0,±8)(0, \pm 8). The xx-coordinate is zero in every case. That tells you the centre of the hyperbola is at the origin (0,0)(0,0), and the transverse axis — the line that goes through the two vertices and the two foci — is the yy-axis.

When the transverse axis is vertical, the standard equation of a hyperbola centred at the origin is:

y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1

Here, aa is the distance from the centre to each vertex, and cc is the distance from the centre to each focus. The relationship between aa, bb, and cc for a hyperbola is c2=a2+b2c^2 = a^2 + b^2. This is different from an ellipse, where c2=a2−b2c^2 = a^2 - b^2 — a common mix-up.

  1. Identify aa and cc from the given points.

    The vertices are (0,±5)(0, \pm 5), so a=5a = 5.

    The foci are (0,±8)(0, \pm 8), so c=8c = 8.

  2. Find b2b^2 using the hyperbola relation.

c2=a2+b2⇒64=25+b2⇒b2=39c^2 = a^2 + b^2 \quad \Rightarrow \quad 64 = 25 + b^2 \quad \Rightarrow \quad b^2 = 39

  1. Write the equation. Since the yy-term comes first (vertical axis), substitute a2=25a^2 = 25 and b2=39b^2 = 39: …

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