Single Choice

Let $$f:\left( 1,3 \right) \rightarrow R$$ be a function defined by $$f(x)=\cfrac { x\left[ x \right] }{ 1+{ x }^{ 2 } } $$, where $$[x]$$ denotes the greatest integer $$\le x$$. Then the range of $$f$$ is:

A$$(\cfrac { 2 }{ 5 } ,\cfrac { 3 }{ 5 } ]\cup \left( \cfrac { 3 }{ 4 } ,\cfrac { 4 }{ 5 } \right) \quad $$
B$$(\cfrac { 2 }{ 5 } ,\cfrac { 4 }{ 5 } ]$$
C$$\left( \cfrac { 2 }{ 5 } ,\cfrac { 1 }{ 2 } \right) \cup (\cfrac { 3 }{ 5 } ,\cfrac { 4 }{ 5 } ]$$
Correct Answer
D$$\left( \cfrac { 3 }{ 5 } ,\cfrac { 4 }{ 5 } \right) $$

Solution

$$f(x) = \dfrac{x [x]}{x^2 + 1} \, : \, (1, 3) \rightarrow R$$ $$f(x) = \begin{cases} \dfrac{x}{x^2 + 1} & x \, \in (1, 2)\\ \dfrac{2x}{x^2 + 1} & x \, \in (2, 3)\end{cases}$$ $$f'(x) = \begin{cases} \dfrac{(x^2 + 1). 1- x.2x}{(x^2 + 1)^2} = \dfrac{1 - x^2}{(x^2 + 1)^2} = - \dfrac{(x^2 - 1)}{(x^2 + 1)^2} \,(-ve) \, \downarrow \\ \dfrac{(x^2 + 1) . 2 - 2x . 2x}{(x^2 + 1)^2} = \dfrac{2 - 2x^2}{(x^2 + 1)^2} = \dfrac{2 - 2x^2}{(x^2 + 1)^2} = \dfrac{-(2x^2 - 2)}{(x^2 + 1)^2} \downarrow \end{cases}$$ $$f(x) $$ is decreasing $$f^n$$. $$f(1) = \dfrac{1}{2}, f(2^-) = \dfrac{2}{5}$$ $$f(2^+) = \dfrac{4}{5} , f(3) = \dfrac{6}{10}$$ $$y \in \left(\dfrac{2}{5} , \dfrac{1}{2} \right) \cup \left(\dfrac{6}{10} , \dfrac{4}{5} \right]$$


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