Let S be the set of all complex numbers z satisfying |z−2+i|≥√5. If the complex number z0 is such that 1|z0−1| is the maximum of the set {1|z−1|:z∈S},then the principal argument of 4−z0−¯¯¯¯¯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 |z−2+i|≥√5, |z0−1|min
⇒P(z0) lies on the line joining AC. Let z0=x+iy (4−z0−¯¯¯¯¯z0z0−¯¯¯¯¯z0+2i)=(4−2x2iy+2i) =2−xi(1+y)=i(x−2)(y+1) From the image, x<1,y>0 x−2y+1<0,4−z0−¯¯¯¯¯z0z0−¯¯¯¯¯z0+2i=−Re(i) ∴arg(4−z0−¯¯¯¯¯z0z0−¯¯¯¯¯z0+2i)=arg(−Re(i))=−π2