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

  • question_answer
    If for complex numbers, \[{{z}_{1}}\]and \[{{z}_{2}}\]arg (\[{{z}_{1}}\]) - arg(\[{{z}_{2}}\])=0 then \[\left| {{z}_{1}}-{{z}_{2}} \right|\]is equal to

    A) \[\left| {{z}_{1}} \right|+\left| {{z}_{2}} \right|\]         

    B) \[\left| {{z}_{1}} \right|-\left| {{z}_{2}} \right|\]

    C) \[\left\| {{z}_{1}}-{{z}_{2}} \right\|\]   

    D) 0

    Correct Answer: C

    Solution :

    [c] We have,
    \[{{\left| {{z}_{1}}-{{z}_{2}} \right|}^{2}}={{\left| {{z}_{1}} \right|}^{2}}+{{\left| {{z}_{2}} \right|}^{2}}-2\left| {{z}_{1}} \right|\left| {{z}_{2}} \right|\cos ({{\theta }_{1}}-{{\theta }_{2}})\]
    Where \[{{\theta }_{1}}\]= are \[({{z}_{1}})\]and \[{{\theta }_{2}}\]=arg \[({{z}_{2}})\].given,
    Arg \[({{z}_{1}}-{{z}_{2}})\]=0.
    \[\Rightarrow {{\left| {{z}_{1}}-{{z}_{2}} \right|}^{2}}={{\left| {{z}_{1}} \right|}^{2}}+{{\left| {{z}_{2}} \right|}^{2}}-2\left| {{z}_{1}} \right|\left| {{z}_{2}} \right|\] \[={{(\left| {{z}_{1}} \right|-\left| {{z}_{2}} \right|)}^{2}}\] \[\Rightarrow \left| {{z}_{1}}-{{z}_{2}} \right|\left. = \right|\left| {{z}_{1}}- \right|\left. \left. {{z}_{2}} \right| \right|\]
     


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