In this explainer, we will learn how to multiply two complex numbers.

The algebra of complex numbers is very similar to the algebra of binomials. Hence, applying the knowledge we have of working with binomials will take us a long way when working with complex numbers. Before we look at multiplication of complex numbers in full generality, we will consider the simpler cases of a complex number multiplied by a real number and a complex number multiplied by a purely imaginary number.

Consider a complex number . If we multiply both sides of this equation by a real number , we get . Using the distributive property, we can rewrite this as much as we would expect.

### Example 1: Multiplying Complex Numbers by Real Numbers

If and , find .

### Answer

Substituting in the values of and into the expression, we get

Expanding the brackets using the distributive property, we get

Finally, we can gather our like terms to find

Having considered the simplest case of multiplying a complex number by a real numbers, we can now consider multiplying a complex number by a purely imaginary number. Taking , we can multiply both sides of the equation by a purely imaginary number to get . Once again, we can use the distributive property to rewrite this as

Since , we can simplify this to

The formulae for multiplying complex numbers by real and imaginary numbers are not formulae that you need to learn. Instead, we should focus on becoming familiar with the algebraic techniques required to work with complex numbers in general.

### Example 2: Multiplying Complex Numbers by Imaginary Numbers

What is ?

### Answer

Expanding the brackets using the distributive property, we can write

Since , we can simplify this to

Let us now consider the product of two general complex numbers and :

Using any technique for multiplying two binomials (FOIL, grid/area, or long multiplication), we can express this as

Using the fact that , and gathering our like terms, we can rewrite this as

We summarize this as follows.

### The Product of Two Complex Numbers

For two complex numbers and , we define the product

Although we have stated a general form, rather than simply memorizing it, it is more important to be familiar with the techniques of multiplying complex numbers.

### Example 3: Multiplying Complex Numbers

Multiply by .

### Answer

Using FOIL, or any other technique, we can expand the brackets as follows:

Since , we can rewrite this as

Finally, gathering our like terms, we have

### Example 4: Squares of Complex Numbers

If and , find .

### Answer

For a question like this, we have a choice: do we multiply out and then substitute in the values of and , or do we do it the other way around? If we try the first method, we will find we have to calculate , , and . This is a lot of computation. However, if we first calculate , we will only need to find the square of the result, which is much more efficient. This is the technique we will use here.

Firstly, we calculate as follows:

To find the square of this number, we can express it as a product and use the technique for multiplying complex numbers. Hence,

Using FOIL or another technique for expanding brackets, we have

Using the fact that , we can gather our like terms and rewrite this as

This example raises the question of what the general form of the square of a complex number is. Using the techniques we have been developing to multiply complex numbers, we can derive a formula for it as follows:

Expanding the brackets, we get

Gathering our like terms, we can state the general form as follows.

### The Square of a Complex Number

For a complex number ,

Even though it is very important to know the techniques needed to derive equations like this, it can also be useful to commit the general form of the square of a complex number to memory. In the next example, we will see how remembering the formula can simplify calculations.

### Example 5: Squares of Complex Numbers

Find .

### Answer

We begin by calculating the square of as follows:

Expanding the brackets, we have

Using the fact that , and gathering like terms, we have

Finally, taking the real part, we get

Alternatively, we could save ourselves some calculation by using the fact that, for a complex number , . Taking the real part, we have

Hence,

### Example 6: Powers of Complex Numbers

If , express in the form .

### Answer

We start by calculating :

Expanding the brackets, we have

To calculate , we can now multiply both sides of the equation by to get Multiplying out the brackets, we get

Clearly, working like this to calculate higher and higher powers of complex numbers could be quite laborious. However, as we learn more about complex numbers we will learn alternative techniques which will significantly simplify the process. We finish by looking at one last example where we can apply what we know about complex numbers and multiplication to solve an equation involving complex numbers.

### Example 7: Solving Equations Involving Complex Numbers

Solve the equation .

### Answer

On first impression, we might think that we need to divide both sides of the equation by to isolate . However, by remembering that , we could simply multiply the whole equation by or, better yet, by .

Hence, by multiplying both sides of the equation by , we get

Expanding the brackets, and simplifying, we arrive at the solution:

It is always good practice to check our answer. To do this, we can multiply our solution by and check that we get back our original equation:

Expanding the brackets and simplifying, we find as required.

### Key Points

- To multiply complex numbers, we use the same techniques that we use to multiply binomials.
- For a complex number ,
- Although, theoretically, using these techniques we can calculate arbitrarily large powers of complex numbers, it requires a large amount of calculation.