I was wondering if someone can point me in the right direction in solving the following complex number problem. The problem is listed below.
The variable complex number z satisfies |z − 2 − i| = 1. Find the maximum and minimum values of |z − 3i|.
Below is my diagram. By using the diagram one can clearly see the result is
$$ 2\sqrt{2} - 1 \leq |z − 3i| \leq 2\sqrt{2} + 1 $$
Where $z_1$ is the minimum value and $z_2$ is the maximum value.
It's the next part that confuses me.
Confirm your answers to the previous part by using the triangle inequality.
$$ \big||z| − |w| \big| ≤ |z + w| ≤ |z| + |w| $$
So... what do I choose for $w$ and $z$?
Any help appreciated.
UPDATE:
Thanks to dxiv below is my solution.
We need to find:
$$ min-value \leq |z - 3i| \leq max-value $$
We also know that $ |z − 2 − i| = 1 $, so can we express $ |z - 3i| $ using $ z − 2 − i $
Thanks to dxiv $ z − 3i = (z − 2 − i )+( 2 − 2i ) $.
Thus
$$ min-value \leq | (z − 2 − i ) + ( 2 − 2i ) | \leq max-value $$
This is in the form
$$ \big||z| − |w| \big| ≤ |z + w| ≤ |z| + |w| $$
$$ \big||z − 2 − i | − |2 − 2i| \big| ≤ |z - 3i| ≤ |z − 2 − i | + | 2 − 2i| $$
$$ \big| 1 − 2\sqrt{2} \big| ≤ |z - 3i| ≤ 1 + 2\sqrt{2} $$
$$ 2\sqrt{2} - 1 \leq |z − 3i| \leq 2\sqrt{2} + 1$$
Thanks again.
