1.

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. \(\rm {(x-1)^2\over 16}-{y^2\over 4} = 1\)

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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