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JEE Main 2023
Conic Sections
Hyperbola
Easy

Question

Let a > 0, b > 0. Let e and l respectively be the eccentricity and length of the latus rectum of the hyperbola x2a2y2b2=1{{{x^2}} \over {{a^2}}} - {{{y^2}} \over {{b^2}}} = 1. Let e' and l' respectively be the eccentricity and length of the latus rectum of its conjugate hyperbola. If e2=1114l{e^2} = {{11} \over {14}}l and (e)2=118l{\left( {e'} \right)^2} = {{11} \over 8}l', then the value of 77a+44b77a + 44b is equal to :

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Solution

Introduction to Hyperbolas and Their Conjugates

This problem requires a solid understanding of the fundamental properties of a hyperbola and its conjugate. Specifically, we need to recall the formulas for their eccentricity and the length of their latus rectum.

  1. Standard Hyperbola (HH): A standard hyperbola with its transverse axis along the x-axis has the equation: x2a2y2b2=1(where a>0,b>0)\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1 \quad (\text{where } a > 0, b > 0)

    • Eccentricity (ee): This value measures how "open" the hyperbola is. For a hyperbola, e>1e > 1. The relationship between a,b,a, b, and ee is given by: e2=1+b2a2e^2 = 1 + \frac{b^2}{a^2}
    • Length of Latus Rectum (ll): The latus rectum is a chord passing through a focus and perpendicular to the transverse axis. Its length is: l=2b2al = \frac{2b^2}{a}
  2. Conjugate Hyperbola (HH'): The conjugate hyperbola to HH has its transverse axis along the y-axis. Its equation is: y2b2x2a2=1\frac{y^2}{b^2} - \frac{x^2}{a^2} = 1 When defining its properties, the semi-transverse axis becomes bb and the semi-conjugate axis becomes aa. This effectively swaps the roles of aa and bb in the formulas for eccentricity and latus rectum compared to the standard hyperbola.

    • Eccentricity (ee'): For the conjugate hyperbola, the relationship between a,b,a, b, and ee' is: (e)2=1+a2b2(e')^2 = 1 + \frac{a^2}{b^2} Self-check: Notice how aa and bb have swapped positions in the fraction compared to the standard hyperbola's eccentricity formula.
    • Length of Latus Rectum (ll'): For the conjugate hyperbola, its length of latus rectum is: l=2a2bl' = \frac{2a^2}{b} Self-check: Similarly, aa and bb have swapped roles in this formula.

Now, let's apply these definitions to solve the given problem step-by-

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