NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 60
Total: 71
Foci (±4, 0), the latus rectum is of length 12.
Solution:  
Here foci are (±4, 0) which lie on x-axis.
So the equation of the hyperbola in standard form is
x2
a
y2
b2
= 1.
Now Foci are (±4, 0) ⇒ c = 4.
Length of latus rectum is
2b2
a
= 12 ⇒ b2 = 6a
We know that c2 = a2+b2
(4)2 = a2 + 6a ⇒ a2 + 6a – 16 = 0
⇒ (a + 8)(a – 2) = 0
⇒ a = 2 (Since a = – 8 is not possible)
a2 = 4 ⇒ b2 = 6 × 2 = 12
Thus, required equation of hyperbola is
x2
4
y2
12
= 1.
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