JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Conjugate, Modulus and Argument of complex number

  • question_answer
    If \[\frac{c+i}{c-i}=a+ib\], where \[a,b,c\]are real, then \[{{a}^{2}}+{{b}^{2}}=\]  [MP PET 1996]

    A) 1

    B) \[-1\]

    C) \[{{c}^{2}}\]

    D) \[-{{c}^{2}}\]

    Correct Answer: A

    Solution :

    \[\frac{c+i}{c-i}=a+ib\] .....(i) \  \[\frac{c+i}{c-i}=a+ib\] .....(ii) Multiplying (i) and (ii), we get \[\frac{{{c}^{2}}+1}{{{c}^{2}}+1}={{a}^{2}}+{{b}^{2}}\] \[\Rightarrow {{a}^{2}}+{{b}^{2}}=1\].


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