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JEE Main 2021
Conic Sections
Hyperbola
Easy

Question

A hyperbola passes through the foci of the ellipse x225+y216=1{{{x^2}} \over {25}} + {{{y^2}} \over {16}} = 1 and its transverse and conjugate axes coincide with major and minor axes of the ellipse, respectively. If the product of their eccentricities is one, then the equation of the hyperbola is :

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Solution

Here is a more elaborate, clear, and educational solution to the problem.


1. Key Concepts and Formulas

To effectively solve this problem, we need a solid understanding of the standard forms and properties of ellipses and hyperbolas centered at the origin. It's crucial to use distinct notations for the parameters of the ellipse and the hyperbola to avoid confusion.

  • Ellipse (Major axis along the x-axis): The standard equation is: x2ae2+y2be2=1where ae>be\frac{x^2}{a_e^2} + \frac{y^2}{b_e^2} = 1 \quad \text{where } a_e > b_e

    • Here, aea_e is the semi-major axis length and beb_e is the semi-minor axis length.
    • Its eccentricity, denoted by eee_e, is given by: ee=1be2ae2or equivalently, be2=ae2(1ee2)e_e = \sqrt{1 - \frac{b_e^2}{a_e^2}} \quad \text{or equivalently, } b_e^2 = a_e^2(1 - e_e^2)
    • Its foci are located at: (±aeee,0)(\pm a_e e_e, 0)
  • Hyperbola (Transverse axis along the x-axis): The standard equation is: x2ah2y2bh2=1\frac{x^2}{a_h^2} - \frac{y^2}{b_h^2} = 1

    • Here, aha_h is the semi-transverse axis length and bhb_h is the semi-conjugate axis length.
    • Its eccentricity, denoted by ehe_h, is given by: eh=1+bh2ah2or equivalently, bh2=ah2(eh21)e_h = \sqrt{1 + \frac{b_h^2}{a_h^2}} \quad \text{or equivalently, } b_h^2 = a_h^2(e_h^2 - 1)
    • Its vertices are located at: (±ah,0)(\pm a_h, 0)
    • Its foci are located at: (±aheh,0)(\pm a_h e_h, 0)

2. Step-by-Step Solution

Let's break down the problem into manageable steps, extracting information from the ellipse first, then applying it to the hyperbola.

Step 1: Analyze the given Ellipse

We are given the equation of the ellipse: x225+y216=1\frac{x^2}{25} + \frac{y^2}{16} = 1

  • Identify ae2a_e^2 and be2b_e^2: By comparing this with the standard form x2ae2+y2be2=1\frac{x^2}{a_e^2} + \frac{y^2}{b_e^2} = 1,

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