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For x > 0, if $$f(x) = \int\limits_1^x {{{{{\log }_e}t} \over {(1 + t)}}dt} $$, then $$f(e) + f\left( {{1 \over e}} \right)$$ is equal to :

JEE · Math · previous-year question

  1. A.$${1 \over 2}$$correct
  2. B.$$-$$1
  3. C.0
  4. D.1

Answer

A. $${1 \over 2}$$

Explanation

$$f(x) = \int_1^x {{{\ln t} \over {1 + t}}dt} $$ then $$f\left( {{1 \over x}} \right) = \int_1^{1/x} {{{\ln t} \over {1 + t}}dt} $$ Let $$t = {1 \over u} \Rightarrow dt = - {1 \over {{u^2}}}du$$ $$ \Rightarrow f\left( {{1 \over x}} \right) = \int_1^x {{{\ln {1 \over u}} \over {1 + {1 \over u}}}\left( { - {1 \over {{u^2}}}} \right)dx} $$ $$f\left( {{1 \over x}} \right) = \int_1^x {{{\ln u} \over {u(1 + u)}}} du = \int_1^x {{{\ln t} \over {t(1 + t)}}dt} $$ $$ \therefore $$ $$f(x) + f\left( {{1 \over x}} \right) = \int_1^x {\ln t\left( {{1 \over {1 + t}} + {1 \over {t(1 + t)}}} \right)} dt$$ $$ = \int_1^x {\ln t\left( {{1 \over {1 + t}} + {1 \over t} - {1 \over {t + 1}}} \right)} dt$$ $$ = \int_1^x {{{\ln t} \over t}dt = {1 \over 2}{{(\ln x)}^2}} $$ $$ \therefore $$ $$f(e) + f\left( {{1 \over e}} \right) = {1 \over 2}{(\ln e)^2} = {1 \over 2}$$

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