JEE Main & Advanced Mathematics Functions Question Bank Self Evaluation Test - Relation and Functions-II

  • question_answer
    Let \[f(z)=sinz\] and \[g(z)=cosz.\] If * denotes a composition of functions, then the value of \[{{(f+ig)}^{*}}(f-ig)\] is:

    A) \[i{{e}^{-{{e}^{-iz}}}}\]

    B) \[i{{e}^{-{{e}^{iz}}}}\]

    C) \[-i{{e}^{-{{e}^{-iz}}}}\]

    D) None of these

    Correct Answer: B

    Solution :

    [b] \[(f-ig)(z)=f(z)-ig(z)=sinz-icosz\] \[=-i(cosz+isinz)=-i{{e}^{iz}}=\theta (say)\] Now \[{{(f+ig)}^{*}}(f-ig)(z)=(f+ig)(f-ig)(z)\] \[=(f+ig)(\theta )=f(\theta )+ig(\theta )=sin\theta +icos\theta \] \[=i(cos\theta -isin\theta )=i{{e}^{-i\theta }}=i{{e}^{-i(-i{{e}^{iz}})}}=i{{e}^{-{{e}^{iz}}}}\]


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