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Proposition: Multiplication of Complex Numbers Using Polar Coordinates
If $z=r\exp(i\phi)$ and $w=s\exp(i\psi)$ are two complex numbers, then their product can be written as $$z\cdot w=(r\cdot s)\cdot [\cos(\phi+\psi)+i\sin(\phi+\psi)].$$
Geometric Construction of the Multiplication of Complex Numbers
The product $z\cdot w$ of two complex numbers $z$ and $w$ can be geometrically constructed as follows:
 Connect the points $z$ and $w$ with the origin of the complex plane and measure the lengths $r$ and $s$ of the created segments $(0,z)$ and $(0,w)$.
 Measure the angles $\phi$ and $\psi$ between the positive $x$axis and the segments $(0,z)$ and $(0,w)$.
 Add the angles $\phi + \psi$ and multiply the lengths $r\cdot s.$
 Draw a circle with radius $r\cdot s$ about the origin of the complex plane and draw a ray at the angle $\phi+\psi$.
 The intersection point between the circle and the ray that is drawn that way is the product of both complex numbers.
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References
Bibliography
 Forster Otto: "Analysis 1, Differential und Integralrechnung einer Veränderlichen", Vieweg Studium, 1983