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NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 56 of 71
Marks: +1, -0
Vertices (0, ±3), foci (0, ±5)
Solution:  
Vertices are (0, ±3) which lie on y-axis.
So the equation of hyperbola in standard form is y2a2x2b2\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1
Now vertices are (0, ±3) ⇒ a = 3; Co-ordinates of Foci are (0, ±5) ⇒ ae = 5
Now, ae = 5 ⇒ e = 5a\frac{5}{a} ⇒ e = 53\frac{5}{3}
We know that b = ae21a\sqrt{e^2-1} ⇒ b = 325913\sqrt{\frac{25}{9} - 1} = 31693\sqrt{\frac{16}{9}} = 4
Thus required equation of hyperbola is y232x242\frac{y^2}{3^2} - \frac{x^2}{4^2} = 1 ⇒ y29x216\frac{y^2}{9} - \frac{x^2}{16} = 1.
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