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NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 55 of 71
Marks: +1, -0
Vertices (0, ±5), foci (0, ±8)
Solution:  
Vertices are (0, ±5) 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, ±5) ⇒ a = 5; Foci are (0, ±8) ⇒ ae = 8
Now, ae = 8 ⇒ e = 8a\frac{8}{a} ⇒ e = 85\frac{8}{5}
We know that b = ae21a\sqrt{e^2-1} ⇒ b = 5642515\sqrt{\frac{64}{25}-1} = 5395\frac{5\sqrt{39}}{5} = 39\sqrt{39}
Thus required equation of parabola is y252x2(39)2\frac{y^2}{5^2} - \frac{x^2}{(\sqrt{39})^2} = 1 ⇒ y225x239\frac{y^2}{25} - \frac{x^2}{39} = 1.
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