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

  • question_answer
    If \[\left| {{z}_{1}} \right|=\left| {{z}_{2}} \right|=.......\left| {{z}_{n}} \right|=1,\] then the value of \[\left| {{z}_{1}}+{{z}_{2}}+....{{z}_{n}} \right|-\left| \frac{1}{{{z}_{1}}}+\frac{1}{{{z}_{2}}}+......+\frac{1}{{{z}_{n}}} \right|\] is,

    A) 0                     

    B) 1

    C) \[-1\]                

    D) None

    Correct Answer: A

    Solution :

    \[{{z}_{1}}{{\bar{z}}_{1}}={{z}_{2}}{{\bar{z}}_{2}}=........={{z}_{n}}{{\bar{z}}_{n}}=1\] \[\Rightarrow \,\,{{\bar{z}}_{1}}=\frac{1}{{{z}_{1}}},{{\bar{z}}_{2}}=\frac{1}{{{z}_{2}}},{{\bar{z}}_{3}}=\frac{1}{{{z}_{3}}},.....,{{\bar{z}}_{n}}=\frac{1}{{{z}_{n}}}\] \[\therefore \,\,\,|{{z}_{1}}+{{z}_{2}}+.......+{{z}_{n}}|-\left| \frac{1}{{{z}_{1}}}+\frac{1}{{{z}_{2}}}+......+\frac{1}{{{z}_{n}}} \right|\] \[\therefore \,\,\,|{{z}_{1}}+{{z}_{2}}+....+{{z}_{n}}|-|{{\bar{z}}_{1}}+{{\bar{z}}_{2}}+....+{{\bar{z}}_{n}}|=0\]


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