If z is any complex number satisfying ∣ z − 3 − 2 i ∣ ≤ 2 then find the maximum value of ∣ 2 z − 6 + 5 i ∣ .
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∣ 2 z − 6 + 5 i ∣ = ∣ 2 z − 6 − 4 i + 9 i ∣ ≤ ∣ 2 z − 6 − 4 i ∣ + ∣ 9 i ∣ = 2 ∣ z − 3 − 2 i ∣ + ∣ 9 i ∣ ≤ 2 . 2 + 9 = 1 3
For two complex numbers z 1 , z 2 we know that ∣ z 1 ∣ + ∣ z 2 ∣ ≥ ∣ z 1 + z 2 ∣
The minimum value of ∣ z − 3 − 2 i ∣ = 2 which is given
The question asks for minimum value, and not maximum. I think the minimum value is 5. @Md Zuhair @Kushal Bose can you recheck?
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Can u post ur solution
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Ok the question has been changed to finding 'maximum' now. Well I used lagrangian multipliers with inequality constraint.
I ask the moderators to change the answer as per Kushal Bose
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∣ z − 3 − 2 i ∣ ∣ x − 3 + ( y − 2 ) i ∣ ( x − 3 ) 2 + ( y − 2 ) 2 ≤ 2 ≤ 2 ≤ 4 Let z = x + y i
We note that y is maximum when x = 3 . That is ( y − 2 ) 2 = 4 ⟹ y m a x = 4 .
Now, consider
∣ 2 z − 6 + 5 i ∣ = 2 ∣ z − 3 + 2 . 5 i ∣ = 2 ( x − 3 ) 2 + ( y + 2 . 5 ) 2 = 2 ( x − 3 ) 2 + ( y − 2 ) 2 − ( y − 2 ) 2 + ( y + 2 . 5 ) 2 ≤ 2 4 − ( y − 2 ) 2 + ( y + 2 . 5 ) 2 ≤ 2 4 − y 2 + 4 y − 4 + y 2 + 5 y + 6 . 2 5 ≤ 2 9 y + 6 . 2 5 ≤ 2 9 y m a x + 6 . 2 5 ≤ 2 4 2 . 2 5 ≤ 1 3