# Polar and cylindrical coordinates/Angle naive/Multiplication in C/Introduction/Section

We discuss several important applications of trigonometric functions like polar coordinates, understanding angles and the trigonometric functions in a naive way.

An angle and a positive real number define a unique point

in the real plane . Here, is the distance between the point and the zero point and means the intersecting point of the ray through with the unit circle. Every point
has a unique representation with
and with an angle , which has to be chosen accordingly
(the zero point is represented by
and an arbitrary angle).
The components are called the *polar coordinates* of .

Every complex number , , can be written uniquely as

with a positive real number , which is the distance between and the zero point
(thus,
)
and an angle between and below degree, measured counterclockwise starting with the positive real axis. The pair constitutes the *polar coordinates* of the complex number.

Polar coordinates in the real plane and for complex numbers are the same. However, the polar coordinates allow a new interpretation of the multiplication of complex numbers: Because of

(where we have used the addition theorems for sine and cosine), one can multiply two complex numbers by multiplying their modulus and adding their angles.

This new way of looking at the multiplication of complex numbers, yields also a new understanding of roots of complex numbers, which exist, due to the fundamental theorem of algebra. If , then

is an -th root of . This means that one has to take the real -th root of the modulus of the complex number and one has to divide the angle by .

A spatial variant of
the polar coordinates
are the so-called *cylindrical coordinates*. A triple
is sent to the
Cartesian coordinates