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Worked Examples · Example 15

Q.Find the equation of the hyperbola with foci (0,±3)(0, \pm 3) and vertices (0,±112)\left(0, \pm \frac{\sqrt{11}}{2}\right).

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The hyperbola is vertical (transverse axis along the y-axis). Using the standard form y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1, we have a=112a = \frac{\sqrt{11}}{2} from the vertices and c=3c = 3 from the foci. Then b2=c2−a2=9−114=254b^2 = c^2 - a^2 = 9 - \frac{11}{4} = \frac{25}{4}, so the equation is y211/4−x225/4=1\frac{y^2}{11/4} - \frac{x^2}{25/4} = 1, or equivalently 4y211−4x225=1\frac{4y^2}{11} - \frac{4x^2}{25} = 1.

The first thing to notice is where the foci and vertices lie. Both are on the y-axis: foci at (0,±3)(0, \pm 3) and vertices at (0,±112)(0, \pm \frac{\sqrt{11}}{2}). That tells you the hyperbola opens upward and downward — its transverse axis is vertical.

For a vertical hyperbola centered at the origin, the standard equation is:

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

Here aa is the distance from the center to each vertex, and cc is the distance from the center to each focus. The relationship between aa, bb, and cc for a hyperbola is c2=a2+b2c^2 = a^2 + b^2 (note: for an ellipse it’s c2=a2−b2c^2 = a^2 - b^2, so don’t mix them up).

  1. Identify aa and cc directly from the given points.

    The vertices are at (0,±112)(0, \pm \frac{\sqrt{11}}{2}), so a=112a = \frac{\sqrt{11}}{2}.

    The foci are at (0,±3)(0, \pm 3), so c=3c = 3.

  2. Use c2=a2+b2c^2 = a^2 + b^2 to find b2b^2.

c2=a2+b2  ⟹  9=114+b2c^2 = a^2 + b^2 \implies 9 = \frac{11}{4} + b^2

b2=9−114=364−114=254b^2 = 9 - \frac{11}{4} = \frac{36}{4} - \frac{11}{4} = \frac{25}{4}

  1. Write the equation. Plug a2=114a^2 = \frac{11}{4} and b2=254b^2 = \frac{25}{4} into the standard form: …

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