JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Geometry of complex numbers

  • question_answer
    The equation \[z\,\overline{z}+a\,\bar{z}+\bar{a}z+b=0,b\in R\] represents a circle if

    A) \[|a{{|}^{2}}=b\]

    B) \[|a{{|}^{2}}>b\]

    C) \[|a{{|}^{2}}<b\]

    D) None of these

    Correct Answer: B

    Solution :

    By adding  \[a\overline{a}\] on both the sides of \[z\overline{z}+a\overline{z}+\overline{a}z=-b\] we get, \[(z+a)(\overline{z}+\overline{a})=a\overline{a}-b\] \[|z+a{{|}^{2}}=\,|a{{|}^{2}}-b,\{\because z\overline{z}=|z{{|}^{2}}\}\] This equation will represent a circle with centre \[z=-a,\]if \[|a{{|}^{2}}-b>0,i.e.|a{{|}^{2}}>b\] since \[|a{{|}^{2}}=b\] represents point circle only.


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