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

  • question_answer
    If  \[|{{z}_{1}}|\,=\,|{{z}_{2}}|\] and \[amp\,{{z}_{1}}+amp\,\,{{z}_{2}}=0\], then [MP PET 1999]

    A) \[{{z}_{1}}={{z}_{2}}\]

    B) \[{{\bar{z}}_{1}}={{z}_{2}}\]

    C) \[{{z}_{1}}+{{z}_{2}}=0\]

    D) \[{{\bar{z}}_{1}}={{\bar{z}}_{2}}\]

    Correct Answer: B

    Solution :

    Let \[{{z}_{1}}={{r}_{1}}(\cos {{\theta }_{1}}+i\sin {{\theta }_{1}})\] Then \[|{{z}_{1}}|\,=\,|{{z}_{2}}|\,\,\Rightarrow |{{z}_{2}}|={{r}_{1}}\] and \[arg({{z}_{1}})+arg({{z}_{2}})=0\]Þ \[arg({{z}_{2}})=-arg({{z}_{1}})=-{{\theta }_{1}}\] \[{{z}_{2}}={{r}_{1}}[\cos (-{{\theta }_{1}})-i\sin (-{{\theta }_{1}})]={{r}_{1}}(\cos {{\theta }_{1}}-i\sin {{\theta }_{1}})\]     \[={{\bar{z}}_{1}}\]\[{{\bar{z}}_{1}}={{z}_{2}}\].


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