Transforming circle in z - plane to w - plane

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A circle in z - plane with diameter $AB, A=(a,0), B=(b,0)$ is transformed to w - plane by transformation $w=\frac{1}{z}$. We need to find the locus of $w$

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$A$ and $B$ in z - plane are transformed to $\left(\frac{1}{a},0\right)$ and $\left(\frac{1}{b},0\right)$ in w - plane respectively. Everywhere on the circle except at $A$ and $B$ we have

$$ Re\left(\frac{z-a}{z-b}\right)=0\rightarrow Re\left(\frac{w-\frac{1}{a}}{w-\frac{1}{b}}\right)=0 $$

Therefore the circle is transformed to a circle in w - plane with diameter $CD, C=\left(\frac{1}{a},0\right), D=\left(\frac{1}{b},0\right)$