BCECE Engineering BCECE Engineering Solved Paper-2005

  • question_answer
    If \[\frac{2{{z}_{1}}}{3{{z}_{2}}}\]is a purely imaginary number, then \[\left| \frac{{{z}_{1}}-{{z}_{2}}}{{{z}_{1}}+{{z}_{2}}} \right|\]is equal to:

    A) \[\frac{3}{2}\]                      

    B)         1                            

    C)  \[\frac{2}{3}\]                     

    D)         \[\frac{4}{9}\]

    Correct Answer: B

    Solution :

    Since, \[\frac{2{{z}_{1}}}{3{{z}_{2}}}\]is a purely imaginary number. Let          \[\frac{2{{z}_{1}}}{3{{z}_{2}}}=ib\] \[\Rightarrow \]    \[\frac{{{z}_{1}}}{{{z}_{2}}}=\frac{3}{2}ib\] Now,       \[\left| \frac{{{z}_{1}}-{{z}_{2}}}{{{z}_{1}}+{{z}_{2}}} \right|=\left| \frac{\frac{{{z}_{1}}}{{{z}_{2}}}-1}{\frac{{{z}_{1}}}{{{z}_{2}}}+1} \right|\]                 \[=\left| \frac{i\frac{3}{2}b-1}{i\frac{3}{2}b+1} \right|\]                 \[=\frac{\sqrt{{{1}^{2}}+{{\left( \frac{3}{2}b \right)}^{2}}}}{{{1}^{2}}+{{\left( \frac{3}{2}b \right)}^{2}}}=1\]


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