BCECE Engineering BCECE Engineering Solved Paper-2009

  • question_answer
    If e and\[{{e}_{1}}\]are the eccentricities of a hyperbola and its conjugate, then\[\frac{1}{{{e}^{2}}}+\frac{1}{e_{1}^{2}}\] is equal to

    A) \[-1\]                    

    B)         0                            

    C)  1                            

    D)         2

    Correct Answer: C

    Solution :

    Let the hyperbola is \[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=1\] Eccentricity, \[e=\sqrt{\frac{{{a}^{2}}+{{b}^{2}}}{{{a}^{2}}}}\] \[\Rightarrow \]               \[\frac{1}{{{e}^{2}}}=\frac{{{a}^{2}}}{({{a}^{2}}+{{b}^{2}})}\] Conjugate hyperbola is \[\frac{{{x}^{2}}}{{{a}^{2}}}-\frac{{{y}^{2}}}{{{b}^{2}}}=-1\] Eccentricity, \[{{e}_{1}}=\sqrt{\frac{{{a}^{2}}+{{b}^{2}}}{{{b}^{2}}}}\] \[\Rightarrow \]               \[\frac{1}{e_{1}^{2}}=\frac{{{b}^{2}}}{{{a}^{2}}+{{b}^{2}}}\] Now, \[\frac{1}{{{e}^{2}}}+\frac{1}{e_{1}^{2}}=\frac{{{a}^{2}}}{{{a}^{2}}+{{b}^{2}}}+\frac{{{b}^{2}}}{{{a}^{2}}+{{b}^{2}}}=1\]


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