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NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 52 of 71
Marks: +1, -0
5y29x25y^2 - 9x^2 = 36
Solution:  
Given equation of hyperbola is 5y29x25y^2 - 9x^2 = 36
i.e., 5y2369x236\frac{5y^2}{36} - \frac{9x^2}{36} = 1 ⇒ y2365x24\frac{y^2}{\frac{36}{5}} - \frac{x^2}{4} = 1,
which is of the form y2a2x2b2\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1.
The foci and vertices of the hyperbola lie on y-axis.
Now, a2a^2 = 365\frac{36}{5} ⇒ a = 65\frac{6}{\sqrt{5}} and b2b^2 = 4 ⇒ b = 2
Also, x2x^2 = a2+b2a^2 + b^2 = 365+4\frac{36}{5} + 4 = 565\frac{56}{5} ⇒ c = 565\sqrt{\frac{56}{5}} = 21452 \sqrt{\frac{14}{5}}
∴ Coordinates of foci are (0, ± c) i.e. (0,±2145)\left(0, \pm \frac{2\sqrt{14}}{\sqrt{5}}\right)
Coordinates of vertices are (0, ± a) i.e. (0,±65)\left(0, \pm \frac{6}{\sqrt{5}}\right)
Eccentricity (e) = ca\frac{c}{a} = 214565\frac{2\sqrt{\frac{14}{5}}}{\frac{6}{\sqrt{5}}} = 143\frac{\sqrt{14}}{3}
Length of latus rectum = 2b2a\frac{2b^2}{a} = 2×465\frac{2 \times 4}{\frac{6}{\sqrt{5}}} = 453\frac{4\sqrt{5}}{3}.
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