February 28, 2020

Chapters

## Expression of a Complex Number in Polar Form

By using the Trigonometric ratios and we can express any Complex number in Polar form. We can do this by rearranging the equations for cosine and sine by saying and and then state a Complex number as

with the modulus or the distance from the origin to the point

and the argument or the directed angle to the vector representing as measured counterclockwise from the positive *x-axis.*

#### Example

and

has and

both and , so the angle is in *Quadrant I* and

then

with

and

(30:60:90 triangle)

#### Example

and

with

then

with

and

(60:30:90 triangle)

#### Example

with

both and are negative so the angle is located in the 3rd Quadrant and we must be careful because the tangent function has period of and the cosine and sine functions have periods of

or

then

with

and

(45:45 in third quadrant)

## Multiplication of the Polar Form and Angle Addition Formulas

Multiplying Complex numbers in Polar form gives insight into how the angle of the Complex number changes in an explicit way.

We know from the section on Multiplication that when we multiply Complex numbers, we multiply the components and their moduli and also add their angles, but the addition of angles doesn't immediately follow from the operation itself. The new Complex number and its modulus do, but the addition of angles needs to be worked out.

Of course, we can see through analysis that this is the case, but it is not until we see Complex numbers in Polar form that we see that it is natural to add the angles.

Here is an explicit proof that angle addition is a result of multiplication:

We need to apply the Trigonometric rules for the addition of angles

and

#### Example

If and then their product is

This shows that multiplying 2 Complex numbers results in the addition of the angles of the numbers.

We can immediately apply the formula to show that the product of a Complex number with itself has a special consequence

#### Example

If with and which is equivalent to

#### Example

with and

## Division of Polar Form and Angle Subtraction Formulas

## The Complex Exponential Function

The exponential number raised to a Complex number is more easily handled when we convert the Complex number to Polar form

where is the Real part and is the radius or modulus and is the Imaginary part with as the argument.

If then becomes $e^{i\theta}=\cos{\theta}+i\sin{\theta}