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NCERT Exemplar · Q32

Q.Find the equation of the hyperbola with:

(a) Vertices (±5,0)(\pm 5, 0), foci (±7,0)(\pm 7, 0);
(b) Vertices (0,±7)(0, \pm 7), e=43e = \dfrac{4}{3};
(c) Foci (0,±10)(0, \pm\sqrt{10}), passing through (2,3)(2, 3).
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We determine the standard form of the hyperbola based on the orientation of its vertices or foci, then use the given parameters (aa, cc, ee) and the relation c2=a2+b2c^2 = a^2 + b^2 to find a2a^2 and b2b^2.

  1. x225−y224=1\frac{x^2}{25} - \frac{y^2}{24} = 1
  2. y249−9x2343=1\frac{y^2}{49} - \frac{9x^2}{343} = 1
  3. y2−x2=5y^2 - x^2 = 5

The equation of a hyperbola depends on the orientation of its transverse axis and the values of its semi-transverse axis (aa) and semi-conjugate axis (bb). The distance from the center to each focus is cc. These parameters are related by c2=a2+b2c^2 = a^2 + b^2.

There are two standard forms for a hyperbola centered at the origin:

  1. Transverse axis along the x-axis: Foci are (±c,0)(\pm c, 0) and vertices are (±a,0)(\pm a, 0). The equation is x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1.
  2. Transverse axis along the y-axis: Foci are (0,±c)(0, \pm c) and vertices are (0,±a)(0, \pm a). The equation is y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1.

Let's solve each part using these concepts.


(a) Vertices (±5,0)(\pm 5, 0), foci (±7,0)(\pm 7, 0)

  1. Identify the orientation and parameters:

    The vertices are (±5,0)(\pm 5, 0) and the foci are (±7,0)(\pm 7, 0). Since both lie on the x-axis, the transverse axis of the hyperbola is along the x-axis.

    From the definition of vertices, a=5a = 5, so a2=25a^2 = 25.

    From the definition of foci, c=7c = 7, so c2=49c^2 = 49.

  2. Find b2b^2 using the fundamental relation:

    The relationship between aa, bb, and cc for a hyperbola is c2=a2+b2c^2 = a^2 + b^2.

    Substitute the values of a2a^2 and c2c^2:

    49=25+b249 = 25 + b^2

    b2=49−25b^2 = 49 - 25

    b2=24b^2 = 24

  3. Write the equation of the hyperbola:

    Since the transverse axis is along the x-axis, the standard form is x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1.

    Substitute a2=25a^2 = 25 and b2=24b^2 = 24:

    x225−y224=1\frac{x^2}{25} - \frac{y^2}{24} = 1


(b) Vertices (0,±7)(0, \pm 7), e=43e = \dfrac{4}{3}

  1. Identify the orientation and parameters:

    The vertices are (0,±7)(0, \pm 7). Since they lie on the y-axis, the transverse axis of the hyperbola is along the y-axis.

    From the definition of vertices, a=7a = 7, so a2=49a^2 = 49.

    The eccentricity is given as e=43e = \frac{4}{3}.

  2. Find cc using eccentricity:

    The eccentricity of a hyperbola is defined as e=cae = \frac{c}{a}.

    We have e=43e = \frac{4}{3} and a=7a = 7.

    43=c7\frac{4}{3} = \frac{c}{7}

    c=4×73=283c = \frac{4 \times 7}{3} = \frac{28}{3}

    So, c2=(283)2=7849c^2 = \left(\frac{28}{3}\right)^2 = \frac{784}{9}.

  3. Find b2b^2 using the fundamental relation:

    Use the relation c2=a2+b2c^2 = a^2 + b^2.

    Substitute a2=49a^2 = 49 and c2=7849c^2 = \frac{784}{9}:

    7849=49+b2\frac{784}{9} = 49 + b^2

    b2=7849−49b^2 = \frac{784}{9} - 49

    To subtract, find a common denominator: 49=49×99=441949 = \frac{49 \times 9}{9} = \frac{441}{9}.

    b2=7849−4419=784−4419=3439b^2 = \frac{784}{9} - \frac{441}{9} = \frac{784 - 441}{9} = \frac{343}{9}

  4. Write the equation of the hyperbola:

    Since the transverse axis is along the y-axis, the standard form is y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1.

    Substitute a2=49a^2 = 49 and b2=3439b^2 = \frac{343}{9}:

    y249−x2343/9=1\frac{y^2}{49} - \frac{x^2}{343/9} = 1

    This can be rewritten as:

    y249−9x2343=1\frac{y^2}{49} - \frac{9x^2}{343} = 1


(c) Foci (0,±10)(0, \pm\sqrt{10}), passing through (2,3)(2, 3)

  1. Identify the orientation and parameters:

    The foci are (0,±10)(0, \pm\sqrt{10}). Since they lie on the y-axis, the transverse axis of the hyperbola is along the y-axis.

    From the definition of foci, c=10c = \sqrt{10}, so c2=10c^2 = 10.

    The hyperbola passes through the point (2,3)(2, 3).

  2. Relate a2a^2 and b2b^2 using c2c^2:

    The fundamental relation is c2=a2+b2c^2 = a^2 + b^2.

    Substitute c2=10c^2 = 10:

    10=a2+b210 = a^2 + b^2

    This means b2=10−a2b^2 = 10 - a^2.

    Watch out

    Remember that a2a^2 and b2b^2 must both be positive. This implies a2<10a^2 < 10.

  3. Form the equation using the passing point: …

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