Q.The points $z_1, z_2, z_3, z_4$ in the complex plane are the vertices of a parallelogram taken in order if and only if :

A complex number $z=x+iy$ can be plotted as the point $(x,y)$, or equivalently as the position vector from the origin $O$ to that point, in the Argand plane (named for Jean Argand) — the $x$-axis is the real axis and the $y$-axis is the imaginary axis. This geometric picture turns algebraic facts about $z$ into statements about points, lines and circles.

Distance and circles. Since $|z_1-z_2|$ is the distance between the points $z_1$ and $z_2$, the equation

is exactly the set of points at distance $r$ from the fixed point $z_0$ — i.e. the complex form of a circle with centre $z_0$ and radius $r$. Correspondingly:

• $|z-z_0|<r$ describes the interior of that circle;
• $|z-z_0|>r$ describes the exterior. An equation like $|\alpha z-\beta|=\gamma$ ($\alpha\ne0$) is first rewritten as $|z-(\beta/\alpha)|=\gamma/|\alpha|$ to read off centre $\beta/\alpha$ and radius $\gamma/|\alpha|$.

Loci from conditions on $z,\overline z$. Many geometric conditions translate directly:

• $|z-a|=|z-b|$ (equidistant from two fixed points) is the perpendicular bisector of the segment joining $a,b$ — e.g. $|z+2|=|z-2|$ gives the imaginary axis, since $\pm2$ are symmetric about it. …