Further Applications of Trigonometry

# Polar Form of Complex Numbers

### Learning Objectives

In this section, you will:

• Plot complex numbers in the complex plane.
• Find the absolute value of a complex number.
• Write complex numbers in polar form.
• Convert a complex number from polar to rectangular form.
• Find products of complex numbers in polar form.
• Find quotients of complex numbers in polar form.
• Find powers of complex numbers in polar form.
• Find roots of complex numbers in polar form.

“God made the integers; all else is the work of man.” This rather famous quote by nineteenth-century German mathematician Leopold Kronecker sets the stage for this section on the polar form of a complex number. Complex numbers were invented by people and represent over a thousand years of continuous investigation and struggle by mathematicians such as Pythagoras, Descartes, De Moivre, Euler, Gauss, and others. Complex numbers answered questions that for centuries had puzzled the greatest minds in science.

We first encountered complex numbers in Complex Numbers. In this section, we will focus on the mechanics of working with complex numbers: translation of complex numbers from polar form to rectangular form and vice versa, interpretation of complex numbers in the scheme of applications, and application of De Moivre’s Theorem.

### Plotting Complex Numbers in the Complex Plane

Plotting a complex numberis similar to plotting a real number, except that the horizontal axis represents the real part of the number,and the vertical axis represents the imaginary part of the number,

### How To

Given a complex numberplot it in the complex plane.

1. Label the horizontal axis as the real axis and the vertical axis as the imaginary axis.
2. Plot the point in the complex plane by movingunits in the horizontal direction andunits in the vertical direction.

### Plotting a Complex Number in the Complex Plane

Plot the complex number in the complex plane.

From the origin, move two units in the positive horizontal direction and three units in the negative vertical direction. See (Figure).

### Try It

Plot the pointin the complex plane.

### Finding the Absolute Value of a Complex Number

The first step toward working with a complex number in polar form is to find the absolute value. The absolute value of a complex number is the same as its magnitude, orIt measures the distance from the origin to a point in the plane. For example, the graph ofin (Figure), shows

### Absolute Value of a Complex Number

Givena complex number, the absolute value ofis defined as

It is the distance from the origin to the point

Notice that the absolute value of a real number gives the distance of the number from 0, while the absolute value of a complex number gives the distance of the number from the origin,

### Finding the Absolute Value of a Complex Number with a Radical

Find the absolute value of

Using the formula, we have

See (Figure).

### Try It

Find the absolute value of the complex number

13

### Finding the Absolute Value of a Complex Number

Givenfind

Using the formula, we have

The absolute valueis 5. See (Figure).

Givenfind

### Writing Complex Numbers in Polar Form

The polar form of a complex number expresses a number in terms of an angleand its distance from the originGiven a complex number in rectangular form expressed aswe use the same conversion formulas as we do to write the number in trigonometric form:

We review these relationships in (Figure).

We use the term modulus to represent the absolute value of a complex number, or the distance from the origin to the pointThe modulus, then, is the same asthe radius in polar form. We useto indicate the angle of direction (just as with polar coordinates). Substituting, we have

### Polar Form of a Complex Number

Writing a complex number in polar form involves the following conversion formulas:

Making a direct substitution, we have

whereis the modulus and is the argument. We often use the abbreviationto represent

### Expressing a Complex Number Using Polar Coordinates

Express the complex numberusing polar coordinates.

On the complex plane, the numberis the same asWriting it in polar form, we have to calculatefirst.

Next, we look atIfandthenIn polar coordinates, the complex numbercan be written asorSee (Figure).

### Try It

Express as in polar form.

### Finding the Polar Form of a Complex Number

Find the polar form of

First, find the value of

Find the angleusing the formula:

### Try It

Writein polar form.

### Converting a Complex Number from Polar to Rectangular Form

Converting a complex number from polar form to rectangular form is a matter of evaluating what is given and using the distributive property. In other words, givenfirst evaluate the trigonometric functionsandThen, multiply through by

### Converting from Polar to Rectangular Form

Convert the polar form of the given complex number to rectangular form:

We begin by evaluating the trigonometric expressions.

After substitution, the complex number is

We apply the distributive property:

The rectangular form of the given point in complex form is[/hidden-answer]

### Finding the Rectangular Form of a Complex Number

Find the rectangular form of the complex number givenand

Ifandwe first determine We then findand

The rectangular form of the given number in complex form is

### Try It

Convert the complex number to rectangular form:

### Finding Products of Complex Numbers in Polar Form

Now that we can convert complex numbers to polar form we will learn how to perform operations on complex numbers in polar form. For the rest of this section, we will work with formulas developed by French mathematician Abraham de Moivre (1667-1754). These formulas have made working with products, quotients, powers, and roots of complex numbers much simpler than they appear. The rules are based on multiplying the moduli and adding the arguments.

### Products of Complex Numbers in Polar Form

Ifand then the product of these numbers is given as:

Notice that the product calls for multiplying the moduli and adding the angles.

### Finding the Product of Two Complex Numbers in Polar Form

Find the product ofgivenand

### Finding Quotients of Complex Numbers in Polar Form

The quotient of two complex numbers in polar form is the quotient of the two moduli and the difference of the two arguments.

### Quotients of Complex Numbers in Polar Form

Ifand then the quotient of these numbers is

Notice that the moduli are divided, and the angles are subtracted.

### How To

Given two complex numbers in polar form, find the quotient.

1. Divide
2. Find
3. Substitute the results into the formula:Replacewithand replacewith
4. Calculate the new trigonometric expressions and multiply through by

### Finding the Quotient of Two Complex Numbers

Find the quotient ofand

Using the formula, we have

### Try It

Find the product and the quotient ofand

### Finding Powers of Complex Numbers in Polar Form

Finding powers of complex numbers is greatly simplified using De Moivre’s Theorem. It states that, for a positive integeris found by raising the modulus to thepower and multiplying the argument byIt is the standard method used in modern mathematics.

### De Moivre’s Theorem

Ifis a complex number, then

where
is a positive integer.

### Evaluating an Expression Using De Moivre’s Theorem

Evaluate the expressionusing De Moivre’s Theorem.

Since De Moivre’s Theorem applies to complex numbers written in polar form, we must first writein polar form. Let us find

Then we findUsing the formulagives

Use De Moivre’s Theorem to evaluate the expression.

### Finding Roots of Complex Numbers in Polar Form

To find the nth root of a complex number in polar form, we use theRoot Theorem or De Moivre’s Theorem and raise the complex number to a power with a rational exponent. There are several ways to represent a formula for findingroots of complex numbers in polar form.

### The nth Root Theorem

To find theroot of a complex number in polar form, use the formula given as

whereWe add toin order to obtain the periodic roots.

### Finding the nth Root of a Complex Number

Evaluate the cube roots of

We have

There will be three roots:Whenwe have

Whenwe have

When we have

Remember to find the common denominator to simplify fractions in situations like this one. Forthe angle simplification is

### Try It

Find the four fourth roots of

Access these online resources for additional instruction and practice with polar forms of complex numbers.

### Key Concepts

• Complex numbers in the formare plotted in the complex plane similar to the way rectangular coordinates are plotted in the rectangular plane. Label the x-axis as the real axis and the y-axis as the imaginary axis. See (Figure).
• The absolute value of a complex number is the same as its magnitude. It is the distance from the origin to the point:See (Figure) and (Figure).
• To write complex numbers in polar form, we use the formulasand Then,See (Figure) and (Figure).
• To convert from polar form to rectangular form, first evaluate the trigonometric functions. Then, multiply through bySee (Figure) and (Figure).
• To find the product of two complex numbers, multiply the two moduli and add the two angles. Evaluate the trigonometric functions, and multiply using the distributive property. See (Figure).
• To find the quotient of two complex numbers in polar form, find the quotient of the two moduli and the difference of the two angles. See (Figure).
• To find the power of a complex numberraise to the power and multiply by See (Figure).
• Finding the roots of a complex number is the same as raising a complex number to a power, but using a rational exponent. See (Figure).

### Section Exercises

#### Verbal

A complex number isExplain each part.

a is the real part, b is the imaginary part, and

What does the absolute value of a complex number represent?

How is a complex number converted to polar form?

Polar form converts the real and imaginary part of the complex number in polar form using and

How do we find the product of two complex numbers?

What is De Moivre’s Theorem and what is it used for?

It is used to simplify polar form when a number has been raised to a power.

#### Algebraic

For the following exercises, find the absolute value of the given complex number.

For the following exercises, write the complex number in polar form.

For the following exercises, convert the complex number from polar to rectangular form.

For the following exercises, findin polar form.

For the following exercises, findin polar form.

For the following exercises, find the powers of each complex number in polar form.

Findwhen

Findwhen

Findwhen

Findwhen

Findwhen

Findwhen

For the following exercises, evaluate each root.

Evaluate the cube root ofwhen

Evaluate the square root ofwhen

Evaluate the cube root ofwhen

Evaluate the square root ofwhen

Evaluate the cube root ofwhen

#### Graphical

For the following exercises, plot the complex number in the complex plane.

#### Technology

For the following exercises, find all answers rounded to the nearest hundredth.

Use the rectangular to polar feature on the graphing calculator to changeto polar form.

Use the rectangular to polar feature on the graphing calculator to change
to polar form.

Use the rectangular to polar feature on the graphing calculator to change
to polar form.

Use the polar to rectangular feature on the graphing calculator to changeto rectangular form.

Use the polar to rectangular feature on the graphing calculator to changeto rectangular form.

Use the polar to rectangular feature on the graphing calculator to changeto rectangular form.

### Glossary

argument
the angle associated with a complex number; the angle between the line from the origin to the point and the positive real axis
De Moivre’s Theorem
formula used to find thepower or nth roots of a complex number; states that, for a positive integeris found by raising the modulus to thepower and multiplying the angles by
modulus
the absolute value of a complex number, or the distance from the origin to the pointalso called the amplitude
polar form of a complex number
a complex number expressed in terms of an angle and its distance from the origincan be found by using conversion formulasand