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Exercise 7.3 · Q76

Q.If e and e' are the eccentricities of a hyperbola and its conjugate hyperbola respectively, prove that 1e2+1e′2=1\dfrac{1}{e^2}+\dfrac{1}{e'^2}=1.

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For the hyperbola x2a2−y2b2=1\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1: e2=1+b2a2=a2+b2a2⇒1e2=a2a2+b2e^2=1+\dfrac{b^2}{a^2}=\dfrac{a^2+b^2}{a^2} \Rightarrow \dfrac{1}{e^2}=\dfrac{a^2}{a^2+b^2}.

For its conjugate y2b2−x2a2=1\dfrac{y^2}{b^2}-\dfrac{x^2}{a^2}=1: e′2=1+a2b2=a2+b2b2⇒1e′2=b2a2+b2e'^2=1+\dfrac{a^2}{b^2}=\dfrac{a^2+b^2}{b^2} \Rightarrow \dfrac{1}{e'^2}=\dfrac{b^2}{a^2+b^2}. …

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