JEE MAIN - Mathematics (2020 - 3rd September Morning Slot - No. 15)
A hyperbola having the transverse axis of
length
$$\sqrt 2 $$ has the same foci as that of the ellipse
3x2 + 4y2 = 12, then this hyperbola does not
pass through which of the following points?
$$\left( {1, - {1 \over {\sqrt 2 }}} \right)$$
$$\left( {\sqrt {{3 \over 2}} ,{1 \over {\sqrt 2 }}} \right)$$
$$\left( { - \sqrt {{3 \over 2}} ,1} \right)$$
$$\left( {{1 \over {\sqrt 2 }},0} \right)$$
Explanation
Ellipse : $${{{x^2}} \over 4} + {{{y^2}} \over 3} = 1$$
eccentricity = $$\sqrt {1 - {3 \over 4}} = {1 \over 2}$$
$$ \therefore $$ foci = ($$ \pm $$ 1, 0)
for hyperbola, given $$2a = \sqrt 2 \Rightarrow a = {1 \over {\sqrt 2 }}$$
$$ \therefore $$ hyperbola will be
$${{{x^2}} \over {1/2}} - {{{y^2}} \over {{b^2}}} = 1$$
eccentricity = $$\sqrt {1 + 2{b^2}} $$
$$ \therefore $$ foci $$ = \left( { \pm \sqrt {{{1 + 2{b^2}} \over 2}} ,0} \right)$$
$$ \because $$ Ellipse and hyperbola have same foci
$$ \Rightarrow \sqrt {{{1 + 2{b^2}} \over 2}} = 1$$
$$ \Rightarrow {b^2} = {1 \over 2}$$
$$ \therefore $$ Equation of hyperbola : $${{{x^2}} \over {1/2}} - {{{y^2}} \over {1/2}} = 1$$
$$ \Rightarrow $$ $${x^2} - {y^2} = {1 \over 2}$$
Clearly $$\left( {\sqrt {{3 \over 2}} ,{1 \over {\sqrt 2 }}} \right)$$ does not lie on it.
eccentricity = $$\sqrt {1 - {3 \over 4}} = {1 \over 2}$$
$$ \therefore $$ foci = ($$ \pm $$ 1, 0)
for hyperbola, given $$2a = \sqrt 2 \Rightarrow a = {1 \over {\sqrt 2 }}$$
$$ \therefore $$ hyperbola will be
$${{{x^2}} \over {1/2}} - {{{y^2}} \over {{b^2}}} = 1$$
eccentricity = $$\sqrt {1 + 2{b^2}} $$
$$ \therefore $$ foci $$ = \left( { \pm \sqrt {{{1 + 2{b^2}} \over 2}} ,0} \right)$$
$$ \because $$ Ellipse and hyperbola have same foci
$$ \Rightarrow \sqrt {{{1 + 2{b^2}} \over 2}} = 1$$
$$ \Rightarrow {b^2} = {1 \over 2}$$
$$ \therefore $$ Equation of hyperbola : $${{{x^2}} \over {1/2}} - {{{y^2}} \over {1/2}} = 1$$
$$ \Rightarrow $$ $${x^2} - {y^2} = {1 \over 2}$$
Clearly $$\left( {\sqrt {{3 \over 2}} ,{1 \over {\sqrt 2 }}} \right)$$ does not lie on it.
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