JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2013

  • question_answer
        If \[{{z}_{1}}=6+i,{{z}_{2}}=4-3i\] and z be a complex number such that arg\[\left( \frac{z-{{z}_{1}}}{{{z}_{2}}-z} \right)=\frac{\pi }{2},\] then z satisfies

    A)  \[|z-(5-i)|=5\] 

    B)  \[|z-(5-i)|=\sqrt{5}\]

    C)  \[|z-(5+i)|=5\]

    D)  \[|z-(5+i)|=\sqrt{5}\]

    Correct Answer: B

    Solution :

                    \[\because \]     \[\arg \left( \frac{z-{{z}_{1}}}{{{z}_{2}}-z} \right)=\frac{\pi }{2}\] \[\Rightarrow \]               \[\operatorname{Re}\left( \frac{z-{{z}_{1}}}{{{z}_{2}}-z} \right)=0\] \[\Rightarrow \]               \[\frac{z-{{z}_{1}}}{{{z}_{2}}-z}+\frac{\overline{z}-{{\overline{z}}_{1}}}{{{\overline{z}}_{2}}-\overline{z}}=0\] \[\Rightarrow \]\[(z-{{z}_{1}})({{\overline{z}}_{2}}-\overline{z})+({{z}_{2}}+-z)(\overline{z}-{{\overline{z}}_{1}})=0\]                \[\Rightarrow \]\[z({{\overline{z}}_{1}}+{{\overline{z}}_{2}})+\overline{z}({{z}_{1}}+{{z}_{2}})2z\overline{z}\]                                                 \[-({{z}_{1}}{{\overline{z}}_{2}}+{{z}_{2}}\overline{{{z}_{1}}})=0\] \[\Rightarrow \]\[z\overline{z}-(5+i)z+(5-i)\overline{z}+21=0\] \[\therefore \]  \[|z-(5-i)|=\sqrt{5}\]


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