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4-1.Complex numbers
medium
If ${z_1},{z_2},{z_3}$ are complex numbers such that $|{z_1}|\, = \,|{z_2}|\, = $ $\,|{z_3}|\, = $ $\left| {\frac{1}{{{z_1}}} + \frac{1}{{{z_2}}} + \frac{1}{{{z_3}}}} \right| = 1\,,$ then${\rm{ }}|{z_1} + {z_2} + {z_3}|$ is
A
Equal to $1$
B
Less than $1$
C
Greater than $3$
D
Equal to $3$
(IIT-2000)
Solution
(a)$1 = \left| {\frac{1}{{{z_1}}} + \frac{1}{{{z_2}}} + \frac{1}{{{z_3}}}} \right|$$ = \left| {\frac{{{z_1}{{\bar z}_1}}}{{{z_1}}} + \frac{{{z_2}{{\bar z}_2}}}{{{z_2}}} + \frac{{{z_3}{{\bar z}_3}}}{{{z_3}}}} \right|$
$(\,\,\,|{z_1}{|^2} = 1 = {z_1}{\overline z _1},{\rm{etc}})$
$ = \,|{\bar z_1} + {\bar z_2} + {\bar z_3}|\, = \,|\overline {{z_1} + {z_2} + {z_3}} |\, = \,|{z_1} + {z_2} + {z_3}|$
$(\because \,\,\,|{\bar z_1}| = |{z_1}|)$
Standard 11
Mathematics