Hard MCQ +4 / -1 PYQ · JEE Mains 2022

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

  1. A 100
  2. B 110
  3. C 120
  4. D 130 Correct answer

Solution

<p>$H:{{{x^2}} \over {{a^2}}} - {{{y^2}} \over {{b^2}}} = 1$, then</p> <p>${e^2} = {{11} \over {14}}l$ (l be the length of LR)</p> <p>$\Rightarrow {a^2} + {b^2} = {{11} \over 7}{b^2}a$ ..... (i)</p> <p>and $e{'^2} = {{11} \over 8}l'$ (l' be the length of LR of conjugate hyperbola)</p> <p>$\Rightarrow {a^2} + {b^2} = {{11} \over 4}{a^2}b$ ....... (ii)</p> <p>By (i) and (ii)</p> <p>$7a = 4b$</p> <p>then by (i)</p> <p>${{16} \over {49}}{b^2} + {b^2} = {{11} \over 7}{b^2}\,.\,{{4b} \over 7}$</p> <p>$\Rightarrow 44b = 65$ and $77a = 65$</p> <p>$\therefore$ $77a + 44b = 130$</p>

About this question

Subject: Mathematics · Chapter: Conic Sections · Topic: Parabola

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