In this explainer, we will learn how to convert a complex number from the algebraic to the exponential form (Eulerβs form) and vice versa.

### Definition: Exponential Form of a Complex Number

A complex number can be written in the form where is the modulus and is the argument expressed in radians.

If we compare the exponential form to the polar form, we find canceling , we arrive at

This formula was derived by the famous mathematician Leonhard Euler (pronounced βOilerβ) and is called Eulerβs formula or Eulerβs relation. In many ways, it is a remarkable formula linking the exponential function , sine, cosine, , and the imaginary unit . We can visualize Eulerβs formula on an Argand diagram as sweeping out a unit circle centered at the origin.

Another way to visualize it is to consider it in three dimensions, where the value of is along one axis and the plane perpendicular to the -axis is the complex plane.

This three-dimensional visualization is closely related to the electrodynamical concept of circular polarization for waves.

Returning to Eulerβs formula, if we set , we get

By adding one to both sides, we derive the famous Eulerβs identity:

This equation is considered by many to be an example of mathematical beauty because by using three of the most fundamental operations in mathematics (addition, multiplication, and exponentiation) just once, it links the five fundamental constants of mathematics: 0, 1, , , and .

We will begin by considering a method to derive Eulerβs relation using power series. We start with the Maclaurin series expansion for :

Substituting into this equation gives

Recall that the integer powers of form a cycle for an integer as follows:

Hence, we have

Gathering the real and imaginary parts separately, we have

The Maclaurin series of sine and cosine are

Hence, we can see

To convert a complex number from algebraic to exponential form, we use a very similar technique as that used for converting between algebraic and trigonometric forms. A summary of how to do this is in the box below.

### How To: Converting a Complex Number from Algebraic Form to Exponential Form

To convert a complex number in algebraic form, , to exponential form:

- Find the modulus, , of the complex number;
- Find the argument, , of the complex number;
- Write the number in exponential form: where and .

### Example 1: Converting Complex Numbers from Algebraic to Exponential Form

Put the number in exponential form.

### Answer

We start by finding the modulus of . Substituting the real and imaginary parts into the formula, we have

Simplifying, we get

We now find the argument of . Notice that since the real part is positive and the imaginary part is negative, lies in the fourth quadrant. Hence, we can find the argument by evaluating the inverse tangent as follows:

Simplifying this fraction gives

Hence, we can express in exponential form as

Given that the argument is -periodic, we could equally add to the argument and express in exponential form as

To convert from exponential form to algebraic form, we rewrite the number in polar form; then, we can convert from this form to algebraic form. In the next example, we will demonstrate this process.

### Definition: Converting from Exponential to Algebraic Form

To convert a complex number to algebraic form, we first convert it to polar form:

Expanding the brackets and evaluating sine and cosine, we arrive at a complex number in algebraic form:

Using the properties of the modulus and argument, we can derive the rules for multiplication and division of complex numbers in polar form. Recall that for two complex numbers, and , their product can also be expressed in exponential form as , where is the modulus and is the argument. Using the properties of the modulus and argument for multiplication, we can see that and . Hence,

Likewise, using the properties of the modulus and argument for division, we can see that

Notice that if we express the argument as an exponential, we can write

Although the previous derivations might give the impression that all of the rules of exponents apply to complex numbers in general, this is, unfortunately, not true. For example, consider the rule , which applies for . If we let and be negative, this rule can no longer be applied in general; for example,

This just goes to demonstrate that we need to be careful when dealing with complex exponents and bases.

We will now look at some examples using the properties of multiplication and division.

### Example 2: Multiplication of Complex Numbers in Exponential Form

Given and , express in the form .

### Answer

Using the multiplicative properties of the exponential form of a complex number, we can write

To convert this to algebraic form, we first express it in polar form as follows:

Expanding the brackets and evaluating sine and cosine, we have

### Example 3: Division of Complex Numbers

Given that , write in exponential form.

### Answer

When faced with a question like this, we have one of two options: we can either convert each number to exponential form and then use their properties to carry out the division, or we can carry out the division with the complex numbers in their current form and then convert the result. Since we have a division to evaluate, it is usually easier to do this with complex numbers in polar form. Hence, we will begin by converting each number to exponential form. Firstly, the numerator is . This is a purely imaginary number; hence, its argument will be . Additionally, its modulus is ; hence, we can express it in exponential form as . As for the denominator, its modulus is , and since it lies in the fourth quadrant, its argument can be calculated by evaluating . Hence, we can express this in exponential form as . Therefore,

Using the properties of division for complex numbers in exponential form, we can rewrite this as

### Example 4: Converting Complex Numbers from Exponential to Algebraic Form

Given that , find the algebraic form of .

### Answer

We begin by separating out the real and imaginary parts of the exponent as follows:

We can now convert this to polar form as follows:

Expanding the brackets and evaluating sine and cosine give us the algebraic form of as follows:

We can also use Eulerβs formula to express sine and cosine in terms of the exponential function as the next example will demonstrate.

### Example 5: Relationship between Sine, Cosine, and the Exponential Function

Use Eulerβs formula to express and in terms of and .

### Answer

We begin by using Eulerβs formula to express in terms of sine and cosine:

Using the even/odd identities for sine and cosine, we can rewrite this as

Adding this to Eulerβs formula, we have

Dividing by 2, we get

Similarly, we can derive a formula by sine by considering the difference of equation (1) with Eulerβs formula as follows:

Dividing by , we arrive at

We will now consider a number of examples where we can use the properties of complex numbers in exponential form to solve problems.

### Example 6: Solving Equations Involving Complex Numbers in Exponential Form

Given that , where and , find and .

### Answer

To solve a problem like this, we want to get the left- and right-hand sides into the same forms. Currently, we have complex numbers in exponential form on the left and a complex number in algebraic form including sine and cosine on the right. We should, therefore, convert the complex number in exponential form to algebraic form. We start by expressing them in polar form:

Using the even/odd trigonometric identities, we can rewrite this as

Expanding the brackets and gathering like terms, we can express this as

We can now equate this with the right-hand side to get

Since we know that , we can equate the real and imaginary parts to get the simultaneous equations

Adding these equations together, we get . Hence, . Substituting this back into one of the equations yields .

### Example 7: Properties of Complex Numbers in Exponential Form

Given , , and , find all possible values of , expressing them in exponential form.

### Answer

We begin by considering what each equation tells us about the value of . Firstly, consider the equation

Using the properties of the modulus, we can rewrite the left-hand side as

Setting this equal to the right-hand side, we have

By dividing both sides by and multiplying by , we get

Hence,

We now consider the equation

What does this tell us? It tells us that is a real number. Consequently, its argument, which we will denote , is either 0 (for a positive real number) or (for a negative real number). Hence, we can write

Using the properties of the argument, we can rewrite this as

Rearranging, we get an expression for the argument of :

We now find the argument of . Since has negative real and imaginary parts, it lies in the third quadrant. Hence, its argument

Therefore, we can write

Now we consider the two cases of and . When , we have

Hence, we can express in exponential form as

When , we have

Hence, we can express in exponential form as

Hence, the two possible values of are

### Example 8: Working with Complex Numbers in Exponential Form

Find the numerical value of .

### Answer

We could approach this problem by converting each number to algebraic form and working through everything. However, we can save ourselves some calculation by being able to recognize a complex conjugate pair when presented in exponential form. Recall that, for any complex number ,

Hence, . Therefore, we have the sum of a complex number and its conjugate. Now if we recall the properties of complex conjugates (i.e., for any complex number, ), we can simplify our expression:

From Eulerβs formula, we know that the real part of . Hence,

### Key Points

- We can express a complex number in exponential form as where is the modulus and is the argument expressed in radians.
- Working with numbers in exponential form can simplify calculations involving multiplication, division, and powers.
- Using Eulerβs formula, we can derive expressions for trigonometric functions such as
- For two complex numbers and ,