JEE Main & Advanced JEE Main Paper (Held on 10-4-2019 Afternoon)

  • question_answer
    If \[\int_{{}}^{{}}{{{x}^{5}}{{e}^{-{{x}^{2}}}}dx}=g(x){{e}^{-{{x}^{2}}}}+c,\]where c is a constant of integration, then g(-1) is equal to: [JEE Main 10-4-2019 Afternoon]

    A) \[-\frac{5}{2}\]          

    B) \[1\]

    C) \[-\frac{1}{2}\]                                  

    D) \[-1\]

    Correct Answer: A

    Solution :

                Let \[{{x}^{2}}=t\]              \[2xdx=dt\]             \[\Rightarrow \frac{1}{2}\int_{{}}^{{}}{{{t}^{2}}.{{e}^{-1}}}dt=\frac{1}{2}\left[ -{{t}^{2}}.{{e}^{-t}}+\int_{{}}^{{}}{2t.{{e}^{-t}}dt} \right]\]             \[=\frac{1}{2}\left( -{{t}^{2}}.{{e}^{-t}} \right)+\left( -t{{e}^{-t}}+\int_{{}}^{{}}{1.{{e}^{-t}}.dt} \right)\]             \[=-\frac{{{t}^{2}}{{e}^{-t}}}{2}-t\,{{e}^{-t}}-\,{{e}^{-t}}=\left( -\frac{{{t}^{2}}}{2}-t-1 \right){{e}^{-t}}\]             \[=\left( -\frac{{{x}^{4}}}{2}-{{x}^{2}}-1 \right){{e}^{-{{x}^{2}}}}+C\]             \[g(x)=-1-{{x}^{2}}-\frac{{{x}^{4}}}{2}+k{{e}^{{{x}^{2}}}}\]for \[k=0\]             \[g(-1)=-1-1-\frac{1}{2}=-\frac{5}{2}\]      


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