JEE MAIN - Mathematics (2018 (Offline) - No. 17)
Let $$f\left( x \right) = {x^2} + {1 \over {{x^2}}}$$ and $$g\left( x \right) = x - {1 \over x}$$,
$$x \in R - \left\{ { - 1,0,1} \right\}$$.
If $$h\left( x \right) = {{f\left( x \right)} \over {g\left( x \right)}}$$, then the local minimum value of h(x) is
$$x \in R - \left\{ { - 1,0,1} \right\}$$.
If $$h\left( x \right) = {{f\left( x \right)} \over {g\left( x \right)}}$$, then the local minimum value of h(x) is
$$2\sqrt 2 $$
3
-3
$$-2\sqrt 2 $$
Explanation
Given $$f\left( x \right) = {x^2} + {1 \over {{x^2}}}$$ and $$g\left( x \right) = x - {1 \over x}$$
As $$h\left( x \right) = {{f\left( x \right)} \over {g\left( x \right)}}$$
= $${{{x^2} + {1 \over {{x^2}}}} \over {x - {1 \over x}}}$$
= $${{{{\left( {x - {1 \over x}} \right)}^2} + 2.x.{1 \over x}} \over {x - {1 \over x}}}$$
= $${{{{\left( {x - {1 \over x}} \right)}^2} + 2} \over {x - {1 \over x}}}$$
Let $${x - {1 \over x} = t}$$
So $$f\left( t \right) = {{{t^2} + 2} \over t}$$ = $$t + {2 \over t}$$
then $$f'\left( t \right) = 1 - {2 \over {{t^2}}}$$
At maximum or minimum $$f'\left( t \right) = 0$$.
$$\therefore$$ $$1 - {2 \over {{t^2}}} = 0$$
$$ \Rightarrow t = \pm \sqrt 2 $$
Now we need to find $$f''\left( t \right)$$ which will tell at which point among $$ + \sqrt 2 $$ and $$ - \sqrt 2 $$ will be maximum and minimum.
$$f''\left( t \right) = + {4 \over {{t^3}}}$$
So when $$t = + \sqrt 2 $$, then $$f''\left( t \right) = + {4 \over {{{\left( {\sqrt 2 } \right)}^3}}}$$ = $$ + \sqrt 2 $$
As $$f''\left( t \right) > 0$$ when $$t = + \sqrt 2 $$ then at $$t = + \sqrt 2 $$ the function $$f\left( t \right)$$ will be minimum.
Min of $$f\left( t \right) = {{{{\left( {\sqrt 2 } \right)}^2} + 2} \over {\sqrt 2 }}$$ = $${4 \over {\sqrt 2 }}$$ = $$2\sqrt 2 $$
So local minimum of $$f\left( t \right)$$ = $$2\sqrt 2 $$
As $$h\left( x \right) = {{f\left( x \right)} \over {g\left( x \right)}}$$
= $${{{x^2} + {1 \over {{x^2}}}} \over {x - {1 \over x}}}$$
= $${{{{\left( {x - {1 \over x}} \right)}^2} + 2.x.{1 \over x}} \over {x - {1 \over x}}}$$
= $${{{{\left( {x - {1 \over x}} \right)}^2} + 2} \over {x - {1 \over x}}}$$
Let $${x - {1 \over x} = t}$$
So $$f\left( t \right) = {{{t^2} + 2} \over t}$$ = $$t + {2 \over t}$$
then $$f'\left( t \right) = 1 - {2 \over {{t^2}}}$$
At maximum or minimum $$f'\left( t \right) = 0$$.
$$\therefore$$ $$1 - {2 \over {{t^2}}} = 0$$
$$ \Rightarrow t = \pm \sqrt 2 $$
Now we need to find $$f''\left( t \right)$$ which will tell at which point among $$ + \sqrt 2 $$ and $$ - \sqrt 2 $$ will be maximum and minimum.
$$f''\left( t \right) = + {4 \over {{t^3}}}$$
So when $$t = + \sqrt 2 $$, then $$f''\left( t \right) = + {4 \over {{{\left( {\sqrt 2 } \right)}^3}}}$$ = $$ + \sqrt 2 $$
As $$f''\left( t \right) > 0$$ when $$t = + \sqrt 2 $$ then at $$t = + \sqrt 2 $$ the function $$f\left( t \right)$$ will be minimum.
Min of $$f\left( t \right) = {{{{\left( {\sqrt 2 } \right)}^2} + 2} \over {\sqrt 2 }}$$ = $${4 \over {\sqrt 2 }}$$ = $$2\sqrt 2 $$
So local minimum of $$f\left( t \right)$$ = $$2\sqrt 2 $$
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