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Question

Let S be the set of all complex numbers z satisfying |z2+i| 5. If the complex number z0 is such that 1|z01| is the maximum of the set {1|z1|:zS},then the principal argument of 4z0¯¯¯¯¯z0z¯¯¯¯¯z0+2i is

A
π2
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B
π4
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C
π2
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D
3π4
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Solution

The correct option is A π2
|z2+i| 5, |z01|min


P(z0) lies on the line joining AC.
Let z0=x+iy
(4z0¯¯¯¯¯z0z0¯¯¯¯¯z0+2i)=(42x2iy+2i)
=2xi(1+y)=i(x2)(y+1)
From the image, x<1, y>0
x2y+1<0, 4z0¯¯¯¯¯z0z0¯¯¯¯¯z0+2i=Re(i)
arg(4z0¯¯¯¯¯z0z0¯¯¯¯¯z0+2i)=arg(Re(i))=π2

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