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NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 60 of 71
Marks: +1, -0
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 x2ay2b2\frac{\frac{x^2}{a^{-y^2}}}{b^2} = 1.
Now Foci are (±4, 0) ⇒ c = 4.
Length of latus rectum is 2b2a\frac{2b^2}{a} = 12 ⇒ b2b^2 = 6a
We know that c2c^2 = a2+b2a^2 + b^2
(4)2(4)^2 = a2a^2 + 6a ⇒ a2a^2 + 6a – 16 = 0
⇒ (a + 8)(a – 2) = 0
⇒ a = 2 (Since a = – 8 is not possible)
a2a^2 = 4 ⇒ b2b^2 = 6 × 2 = 12
Thus, required equation of hyperbola is x24y212\frac{x^2}{4} - \frac{y^2}{12} = 1.
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