The equation arg

Question:

The equation $\arg \left(\frac{\mathrm{z}-1}{\mathrm{z}+1}\right)=\frac{\pi}{4}$ represents a circle with:

  1. centre at $(0,-1)$ and radius $\sqrt{2}$

  2. centre at $(0,1)$ and radius $\sqrt{2}$

  3. centre at $(0,0)$ and radius $\sqrt{2}$

  4. centre at $(0,1)$ and radius 2


Correct Option: 2,

Solution:

In $\Delta \mathrm{OAC}$

$\sin \left(\frac{\pi}{4}\right)=\frac{1}{\mathrm{AC}}$

$\Rightarrow \mathrm{AC}=\sqrt{2}$

Also, $\tan \frac{\pi}{4}=\frac{\mathrm{OA}}{\mathrm{OC}}=\frac{1}{\mathrm{OC}}$

$\Rightarrow \mathrm{OC}=1$

$\therefore$ centre $(0,1) ;$ Radius $=\sqrt{2}$

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