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

  • question_answer
    If \[\omega \] is a complex cube root of unity and \[x={{\omega }^{2}}-\omega -2,\] then what is the value of \[{{x}^{2}}+4x+7\]?

    A) \[-2\]    

    B) \[-1\]  

    C) \[0\]     

    D) \[1\]

    Correct Answer: C

    Solution :

    Given \[\operatorname{x}={{\omega }^{2}}-\omega -2\] \[\Rightarrow \,\,\,x+2={{\omega }^{2}}-\omega \] On squaring both sides, we get \[\Rightarrow \,\,\,{{(x+2)}^{2}}=\,\,{{\left( {{\omega }^{2}}-\omega  \right)}^{2}}\] \[\Rightarrow \,\,\,\,{{\operatorname{x}}^{2}} + 4x + 4 = {{\omega }^{4}} + {{\omega }^{2}}- 2 {{\omega }^{3}}\] Add 3 on both side \[\Rightarrow \,\,{{\operatorname{x}}^{2}}+4x+4+3=\omega +{{\omega }^{2}}-2+3\,\,\,\left( \because \,\,{{\omega }^{3}}=1 \right)\] \[{{\operatorname{x}}^{2}} + 4x + 7 = 1 + \omega  + {{\omega }^{2}}\] \[\Rightarrow \,\,\,{{\operatorname{x}}^{2}} +4x\times 4-7 =0\,\,\left( \because \,\, 1+\omega +{{\omega }^{2}} =0 \right)\]


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