NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 62
Total: 71
Foci (0, ± 10) , passing through (2, 3).
Solution:  
Here foci are (0, ± 10) which lie on y-axis.
So the equation of hyperbola in standard form is
y2
a2
x2
b2
= 1
Now, foci are (0, ± 10) ⇒ c = 10
We know that c2 = a2+b2
(10)2 = a2+b2b2 = 10 - a2
Since the hyperbola passes through (2, 3)
9
a2
4
b2
= 1 ⇒
9
a2
4
10a2
= 1
⇒ 9(10 − a2) − 4a2a2(10a2) = 0
a423a2 + 90 = 0 ⇒ a418a25a2 + 90 = 0
⇒ (a2 – 18)(a2 – 5) = 0 ⇒ a2 = 18 or a2 = 5
When a2 = 18 then b2 = 10 – 18 = –8 (which is not possible)
When a2 = 5, then b2 = 10 – 5 = 5
Thus, required equation of hyperbola is
y2
5
x2
5
= 1.
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