InterviewSolution
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Equation of the hyperbola whose foci are (5,0) and (-3,0), eccentricity = 2, is1. \(\rm {x^2\over 4}-{y^2\over 12} = 1\)2. \(\rm {(x-1)^2\over 4}-{y^2\over 12} = 1\)3. \(\rm {(x-1)^2\over 4}-{y^2\over 16} = 1\)4. \(\rm {(x-1)^2\over 16}-{y^2\over 4} = 1\) |
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Answer» Correct Answer - Option 2 : \(\rm {(x-1)^2\over 4}-{y^2\over 12} = 1\) Concept: The standard equation of a hyperbola: \(\rm {(x-h)^2\over a^2}-{(y-k)^2\over b^2} = 1\) where 2a and 2b are the length of the transverse axis and conjugate axis respectively and centre (h, k) Note: The centre is the midpoint of the 2 foci. The eccentricity = \(\rm \sqrt{a^2+b^2}\over a\) Length of latus recta = \(\rm 2b^2 \over a\) Distance from center to focus = \(\rm \sqrt{a^2+b^2}\) Calculation: Given foci are (5,0) and (-3,0) Center = \(\rm \left({5+(-3)\over2},{0+0\over2}\right)\) = (1, 0) Now distance of focus from the center = \(\rm \sqrt{a^2+b^2}\) ⇒ \(\rm \sqrt{(5-1)^2+(0-0)^2} = \rm \sqrt{a^2+b^2}\) ⇒ a2 + b2 = 16 ...(i) The eccentricity = \(\rm \sqrt{a^2+b^2}\over a\) ⇒ 2 = \(\rm \sqrt{16}\over a\) ⇒ a = 2 Putting it in (i) ⇒ 4 + b2 = 16 ⇒ b2 = 12 a2 = 4 The equation of the hyperbola \(\rm {(x-h)^2\over a^2}-{(y-k)^2\over b^2} = 1\) ⇒ \(\rm {(x-1)^2\over 4}-{(y-0)^2\over 12} = 1\) ⇒ \(\boldsymbol{\rm {(x-1)^2\over 4}-{y^2\over 12} = 1}\) |
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