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माना दीर्घवृत्त, $\frac{ x ^{2}}{25}+\frac{ y ^{2}}{ b ^{2}}=1( b <5)$ तथा अतिपरवलय $\frac{x^{2}}{16}-\frac{y^{2}}{b^{2}}=1$ की उत्केन्द्रताएँ क्रमशः $e_{1}$ तथा $e_{2}$ है और $e_{1} e_{2}$ $=1$ है। यदि दीर्घवृत्त और अतिपरवलय के नाभिकेन्दों के बीच की दूरीयाँ क्रमशः $\alpha$ तथा $\beta$ हैं, तो क्रमित युग्म $(\alpha, \beta)$ बराबर है
$(8,10)$
$(8,12)$
$\left(\frac{20}{3}, 12\right)$
$\left(\frac{24}{5}, 10\right)$
Solution
For ellipse $\frac{x^{2}}{25}+\frac{y^{2}}{b^{2}}=1 \quad(b<5)$
Let e $_{1}$ is eccentricity of ellipse
$\therefore \quad b^{2}=25\left(1-e_{1}^{2}\right) \ldots \ldots .(1)$
Again for hyperbola
$\frac{x^{2}}{16}-\frac{y^{2}}{b^{2}}=1$
Let $e _{2}$ is eccentricity of hyperbola.
$\therefore \quad b^{2}=16\left(e_{2}^{2}-1\right) \quad \ldots \ldots$
by (1)$\&(2)$
$25\left(1- e _{1}^{2}\right)=16\left( e _{2}^{2}-1\right)$
Now $e _{1} e _{2}=1 \quad$ (given)
$\therefore \quad 25\left(1- e _{1}^{2}\right)=16\left(\frac{1- e _{1}^{2}}{ e _{1}^{2}}\right)$
or $\quad e _{1}=\frac{4}{5} \quad \therefore e _{2}=\frac{5}{4}$
Now distance between foci is $2 ae$
$\therefore$ distance for ellipse $=2 \times 5 \times \frac{4}{5}=8=\alpha$
distance for hyperbola $=2 \times 4 \times \frac{5}{4}=10=\beta$
$\therefore(\alpha, \beta) \equiv(8,10)$