Test Index

NCERT Class XI Mathematics - Conic Sections - Solutions

© examsnet.com
Question : 62 of 71
Marks: +1, -0
Foci (0, ± 10\sqrt{10}) , passing through (2, 3).
Solution:  
Here foci are (0, ± 10\sqrt{10}) 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, foci are (0, ± 10\sqrt{10}) ⇒ c = 10\sqrt{10}
We know that c2c^2 = a2+b2a^2 + b^2
(10)2(\sqrt{10})^2 = a2+b2a^2+b^2b2b^2 = 10 - a2a^2
Since the hyperbola passes through (2, 3)
9a24b2\frac{9}{a^2} - \frac{4}{b^2} = 1 ⇒ 9a2410a2\frac{9}{a^2} - \frac{4}{10-a^2} = 1
⇒ 9(10 − a2a^2) − 4a24a^2a2(10a2)a^2(10 - a^2) = 0
a423a2a^4 - 23a^2 + 90 = 0 ⇒ a418a25a2a^4 - 18a^2 - 5a^2 + 90 = 0
⇒ (a2a^2 – 18)(a2a^2 – 5) = 0 ⇒ a2a^2 = 18 or a2a^2 = 5
When a2a^2 = 18 then b2b^2 = 10 – 18 = –8 (which is not possible)
When a2a^2 = 5, then b2b^2 = 10 – 5 = 5
Thus, required equation of hyperbola is y25x25\frac{y^2}{5} - \frac{x^2}{5} = 1.
© examsnet.com
Go to Question: