JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Self Evaluation Test - Complex Numbers and Quadratic Equations

  • question_answer
    The value of \[Arg\left[ i\,\,\ln \left( \frac{a-ib}{a+ib} \right) \right],\] where a and b are real numbers, is

    A) \[0\] or \[\pi \] 

    B) \[\frac{\pi }{2}\]

    C) not defined     

    D) None of these

    Correct Answer: A

    Solution :

    \[\ell n\left( \frac{a-ib}{a+ib} \right)=\ell n\left| \frac{a-ib}{a+ib} \right|+i\left[ 2n\pi +\arg \left( \frac{a-ib}{a+ib} \right) \right]\] \[=\,\,\,i\left[ 2n\pi +\arg \left( \frac{a-ib}{a+ib} \right) \right]\,Since\,\,\left| \frac{a-ib}{a+ib} \right|=1\] \[\therefore \,\,\,\,Arg\left[ i\ell n\left( \frac{a-ib}{a+ib} \right) \right]\] \[=\,\,Arg\left[ -2n\pi -\arg \left( \frac{a-ib}{a+ib} \right) \right]=0\,\,or\,\,\pi \] As \[2n\pi  + arg\left( \frac{a-ib}{a+ib} \right)\] is a real number.


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