The study of mathematics continuously builds upon itself. Negative integers, for example, fill a void left by the set of positive integers. The set of rational numbers, in turn, fills a void left by the set of integers. The set of real numbers fills a void left by the set of rational numbers. The set of real numbers has voids as well. For example, we still have no solution to equations such as

${x}^{2}+4=0$

Our best guesses might be +2 or –2. But if we test +2 in this equation, it does not work. If we test –2, it does not work. If we want to have a solution for this equation, we will have to go farther than we have so far. After all, to this point we have described the square root of a negative number as undefined. Fortunately there is another system of numbers that provides solutions to problems such as these. In this section we will explore this number system and how to work within it.

Express and Plot Complex Numbers

We know how to find the square root of any positive real number. In a similar way we can find the square root of a negative number. The difference is that the root is not real. If the value in the radicand is negative, the root is said to be an imaginary number. The imaginary number $i$ is defined as the square root of negative 1.

$\sqrt{-1}=i$

So, using properties of radicals,

${i}^{2}={\left(\sqrt{-1}\right)}^{2}=-1$

We can write the square root of any negative number as a multiple of $i$. Consider the square root of –25.

$\begin{align}\sqrt{-25}&=\sqrt{25\cdot \left(-1\right)}\\&=\sqrt{25}\cdot\sqrt{-1}\\ &=5i\end{align}$

We use $5i$ and not $-\text{5}i$ because the principal root of 25 is the positive root.

A complex number is the sum of a real number and an imaginary number. A complex number is expressed in standard form when written $a+bi$ where $a$ is the real part and $bi$ is the imaginary part. For example, $5+2i$ is a complex number. So, too, is $3+4\sqrt{3}i$.

Imaginary numbers are distinguished from real numbers because a squared imaginary number produces a negative real number. Recall, when a positive real number is squared, the result is a positive real number and when a negative real number is squared, again, the result is a positive real number. Complex numbers are a combination of real and imaginary numbers.

Plot complex numbers on the complex plane

We cannot plot complex numbers on a number line as we might real numbers. However, we can still represent them graphically. To represent a complex number we need to address the two components of the number. We use the complex plane, which is a coordinate system in which the horizontal axis represents the real component and the vertical axis represents the imaginary component. Complex numbers are the points on the plane, expressed as ordered pairs $(a, b)$, where $a$ represents the coordinate for the horizontal axis and $b$ represents the coordinate for the vertical axis.

Let’s consider the number $-2+3i$. The real part of the complex number is $–2$ and the imaginary part is $3i$. We plot the ordered pair $\left(-2,3\right)$ to represent the complex number $-2+3i$.

Add, Subtract, and Multiply Complex Numbers

Just as with real numbers, we can perform arithmetic operations on complex numbers. To add or subtract complex numbers, we combine the real parts and combine the imaginary parts.

Multiplying Complex Numbers

Multiplying complex numbers is much like multiplying binomials. The major difference is that we work with the real and imaginary parts separately.

Multiplying a Complex Number by a Real Number

Let’s begin by multiplying a complex number by a real number. We distribute the real number just as we would with a binomial. So, for example,

$\begin{align}3(6+2i)&=(3\cdot6)+(3\cdot2i)&&\text{Distribute.}\\&=18+6i&&\text{Simplify.}\end{align}$

Multiplying Complex Numbers Together

Now, let’s multiply two complex numbers. We can use either the distributive property or the FOIL method. Recall that FOIL is an acronym for multiplying First, Outer, Inner, and Last terms together. Using either the distributive property or the FOIL method, we get

$\left(a+bi\right)\left(c+di\right)=ac+adi+bci+bd{i}^{2}$

Because ${i}^{2}=-1$, we have

$\left(a+bi\right)\left(c+di\right)=ac+adi+bci-bd$

To simplify, we combine the real parts, and we combine the imaginary parts.

$\left(a+bi\right)\left(c+di\right)=\left(ac-bd\right)+\left(ad+bc\right)i$

Divide Complex Numbers

Division of two complex numbers is more complicated than addition, subtraction, and multiplication because we cannot divide by an imaginary number, meaning that any fraction must have a real-number denominator. We need to find a term by which we can multiply the numerator and the denominator that will eliminate the imaginary portion of the denominator so that we end up with a real number as the denominator. This term is called the complex conjugate of the denominator, which is found by changing the sign of the imaginary part of the complex number. In other words, the complex conjugate of $a+bi$ is $a-bi$.

Note that complex conjugates have a reciprocal relationship: The complex conjugate of $a+bi$ is $a-bi$, and the complex conjugate of $a-bi$ is $a+bi$. Importantly, complex conjugate pairs have a special property. Their product is always real.

$\begin{align}(a+bi)(a-bi)&=a^2-abi+abi-b^2i^2\\[2mm]&=a^2-b^2(-1)\\[2mm]&=a^2+b^2\end{align}$

Suppose we want to divide $c+di$ by $a+bi$, where neither $a$ nor $b$ equals zero. We first write the division as a fraction, then find the complex conjugate of the denominator, and multiply.

$\dfrac{c+di}{a+bi}$ where $a\ne 0$ and $b\ne 0$.

Multiply the numerator and denominator by the complex conjugate of the denominator.

$\dfrac{\left(c+di\right)}{\left(a+bi\right)}\cdot \dfrac{\left(a-bi\right)}{\left(a-bi\right)}=\dfrac{\left(c+di\right)\left(a-bi\right)}{\left(a+bi\right)\left(a-bi\right)}$

Apply the distributive property.

$=\dfrac{ca-cbi+adi-bd{i}^{2}}{{a}^{2}-abi+abi-{b}^{2}{i}^{2}}$

Simplify, remembering that ${i}^{2}=-1$.

$\begin{align}&=\dfrac{ca-cbi+adi-bd\left(-1\right)}{{a}^{2}-abi+abi-{b}^{2}\left(-1\right)} \\[2mm] &=\dfrac{\left(ca+bd\right)+\left(ad-cb\right)i}{{a}^{2}+{b}^{2}}\end{align}$

Simplifying Powers of $i$

The powers of $i$ are cyclic. Let’s look at what happens when we raise $i$ to increasing powers.

${i}^{1}=i$
${i}^{2}=-1$
${i}^{3}={i}^{2}\cdot i=-1\cdot i=-i$
${i}^{4}={i}^{3}\cdot i=-i\cdot i=-{i}^{2}=-\left(-1\right)=1$
${i}^{5}={i}^{4}\cdot i=1\cdot i=i$

We can see that when we get to the fifth power of $i$, it is equal to the first power. As we continue to multiply $i$ by itself for increasing powers, we will see a cycle of 4. Let’s examine the next 4 powers of $i$.

${i}^{6}={i}^{5}\cdot i=i\cdot i={i}^{2}=-1$
${i}^{7}={i}^{6}\cdot i={i}^{2}\cdot i={i}^{3}=-i$
${i}^{8}={i}^{7}\cdot i={i}^{3}\cdot i={i}^{4}=1$
${i}^{9}={i}^{8}\cdot i={i}^{4}\cdot i={i}^{5}=i$

Key Concepts

• The square root of any negative number can be written as a multiple of $i$.
• To plot a complex number, we use two number lines, crossed to form the complex plane. The horizontal axis is the real axis, and the vertical axis is the imaginary axis.
• Complex numbers can be added and subtracted by combining the real parts and combining the imaginary parts.
• Complex numbers can be multiplied and divided.
• To multiply complex numbers, distribute just as with polynomials.
• To divide complex numbers, multiply both the numerator and denominator by the complex conjugate of the denominator to eliminate the complex number from the denominator.
• The powers of $i$ are cyclic, repeating every fourth one.

Glossary

complex conjugate

the complex number in which the sign of the imaginary part is changed and the real part of the number is left unchanged; when added to or multiplied by the original complex number, the result is a real number

complex number

the sum of a real number and an imaginary number, written in the standard form $a+bi$, where $a$ is the real part, and $bi$ is the imaginary part

complex plane

a coordinate system in which the horizontal axis is used to represent the real part of a complex number and the vertical axis is used to represent the imaginary part of a complex number

imaginary number

a number in the form $bi$ where $i=\sqrt{-1}\\$