RedCrab Math Tutorial

# Complex numbers

## introduction

Why complex numbers?

In the article natural up to real numbers, the extension from natural to integer numbers is described, which has been extended to rational and then to real numbers. Many tasks could only be solved with real numbers.

But even with the real numbers, not all tasks can be solved. For example, the equation

x2 + 1 = 0 or x2 = -1

The equation is not solvable with real numbers because the square of a real number not equal to zero is always positive.

In order to be able to solve such equations, one introduced a new number, an imaginary number and designated these with the letter i.

## The imaginary part i

In the introduction it has already been stated that the equation x2 = -1 with the set of real numbers is not solvable because the square of a real number not equal to zero is always positive.

So, we need to expand the range of real numbers so that the equation is solvable.

For this we need a new number, which makes the equation solvable. This new number is called imaginary part and is denoted by the symbol i. It has the property that if it multiplies by itself the result -1.

i ∙ i = -1

What can one imagine under i? Certainly i is not a real number, because the square of a real number is never negative. But i2 = -1, because the requirement was a solution of the equation x2 = -1.

Although it is not yet clear what i looks like, we can already calculate with i.

The calculation rules of real numbers should continue to be valid even for complex numbers. This is called principle of permanence. So we can also calculate i3

i3 = i2 ∙ i = (-1) ∙ i = -i

## Complex numbers

What is a complex number?

The word complex is derived from the Latin word complexus = intertwined. A complex number is the connection of the imaginary part i with a real number.

Examples of complex numbers:

1 + 3i,              -1 + 3i              2-3i                  22 – 52i

The numbers are all composed of a real part and an imaginary part. For example, 1 + 3i this is the real number 1 and the imaginary part 3i.

The + sign in 1 + 3i is part of the complex number. It will later be considered as a addition. The part 3i is understood as 3 ∙ i, finally the calculation rules of the real numbers for the numbers 3 and i continue to be valid

A complex number is defined as

z = a + bi

This term is a complex number. a and b are real numbers and i stands for the imaginary part.

Another notation for complex numbers is the pair spelling: The real and imaginary part is written as a pair of numbers:

z = (Re, Im)

For z = 2 + 3i this would be  z = (2, 3). For the real number 5, the pair notation z = (5, 0)  and for 2i we write  z = (0, 2).

## Terms of complex numbers

As an example we take the complex number 2 - 5i. The complex number 2 - 5i is uniquely determined by the real numbers 2 and -5. Generally, any complex number a + bi is uniquely defined by the real numbers a and b.

To describe complex numbers in this way, we introduced two new terms:

The real part of a complex number is the purely real part of the number. The real part of the complex number 2 - 5i is therefore 2. We also write Re (2 - 5i) = 2.

The imaginary part of a complex number is the part of the number that precedes the imaginary part i. The imaginary part of the complex number 2 - 5i is thus -5. We also writes Im (2 - 5i) = -5.

### Summary

A complex number is defined as , where a and b are real numbers

The real part of z is called a              We write a = Re (z)

The imaginary part of z is called     We write b = Im (z)

## Complex numbers as an extension of the real numbers

Like the expansion from natural to integer numbers, which have been extended to rational and then to real numbers, complex numbers are an extension of real numbers. Real numbers are thus a subset of the complex numbers. A real number is identical to a complex number with the imaginary part 0.

Since a complex number consists of a pair of numbers Re and Im, it can neither be represented as real numbers on a number-ray nor can complex numbers be compared with each other as greater than, or less than.

However, due to the number pair, complex numbers can be represented in a special coordinate system - a complex plane, the Gaussian plane of numbers. The real part here corresponds to the x-coordinate, the imaginary part of the y-coordinate. Read more about this in the article on the geometric representation of complex numbers.

## Complex numbers

Basics of complex numbers
Computing with complex numbers
Multiplication of complex numbers
Conjugate complex numbers
Division of complex numbers
Geometric representation
Absolute value of a complex number
The polar form of complex numbers
Calculate polar form
Convert polar form to normal
Multiplication in polar form
Division in polar form