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Arithmetic With Complex Numbers

Learning Outcomes

• Identify the difference between an imaginary number and a complex number
• Identify the real and imaginary parts of a complex number
• Plot a complex number on the complex plane
• Perform arithmetic operations on complex numbers
• Graph physical representations of arithmetic operations on complex numbers as scaling or rotation

Complex Numbers

[footnote]Portions of this section are remixed from Precalculus: An Investigation of Functions by David Lippman and Melonie Rasmussen. CC-BY-SA[/footnote] The numbers you are most familiar with are called real numbers. These include numbers like 4, 275, -200, 10.7, ½, π, and so forth. All these real numbers can be plotted on a number line. For example, if we wanted to show the number 3, we plot a point on the number line equidistant between 2 and 4. To solve certain problems like $x^{2}=–4$, it became necessary to introduce imaginary numbers.

Recall Square roots

Recall that the square root of a number $\sqrt{n}$ is another way of asking the question what number when multiplied by itself results in the number $n$? Ex. $\sqrt{9}=3$ because $3 \ast 3 = 9$. It is also true that $(-3)\ast (-3) = 9$ although we agreed that using the radical symbol requests only the principle root, the positive one. But, there is no number that when multiplied by itself results in a negative number. The property of integer multiplication states that both a negative number squared and a positive number squared result in a positive number. Indeed, you saw in the review section to this module that the square root of a negative number does not exist in the set of real numbers. Mathematicians realized the helpfulness of being to do calculations with such numbers though, so they assigned a value to $\sqrt{-1}$, calling it the imaginary unit $i$. In this section, you'll use what you know about simplifying and doing operations on square roots and binomials to perform arithmetic on complex numbers (numbers that are the sum of a real and an imaginary number).

Imaginary Number i

The imaginary number $i$ is defined to be $i=\sqrt{-1}$. Any real multiple of $i$, like 5$i$, is also an imaginary number.

Example

Simplify $\sqrt{-9}$. We can separate $\sqrt{-9}$ as $\sqrt{9}\sqrt{-1}$. We can take the square root of 9, and write the square root of $-1$ as i.

$\sqrt{-9}=\sqrt{9}\sqrt{-1}=3i$

A complex number is the sum of a real number and an imaginary number.

The form of a complex number

The form of a complex number may look familiar to you. It looks a little like a binomial in one variable, doesn't it? Ex. $a + bi$ is a complex number. And $(a + bx)$ is a binomial. You may be wondering about arithmetic on complex numbers -- does it behave similarly to arithmetic on binomial expressions? Good question! The answer is yes, it does.

Complex Number

A complex number is a number $z=a+bi$, where
• $a$ and $b$ are real numbers
• $a$ is the real part of the complex number
• $b$ is the imaginary part of the complex number
To plot a complex number like $3-4i$, we need more than just a number line since there are two components to the number. To plot this number, we need two number lines, crossed to form a complex plane.

Complex Plane

In the complex plane, the horizontal axis is the real axis and the vertical axis is the imaginary axis.

Example

Plot the number $3-4i$ on the complex plane. The real part of this number is 3, and the imaginary part is $-4$. To plot this, we draw a point 3 units to the right of the origin in the horizontal direction and 4 units down in the vertical direction.

Try It

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Because this is analogous to the Cartesian coordinate system for plotting points, we can think about plotting our complex number $z=a+bi$ as if we were plotting the point (a, b) in Cartesian coordinates. Sometimes people write complex numbers as $z=x+yi$ to highlight this relation.

Arithmetic on Complex Numbers

Before we dive into the more complicated uses of complex numbers, let’s make sure we remember the basic arithmetic involved. To add or subtract complex numbers, we simply add the like terms, combining the real parts and combining the imaginary parts.

Multiplying complex numbers

Multiplying complex numbers is algebraically similar to multiplying binomial expressions. Can you recognize the FOIL process in the example below?

Example

Add $3-4i$ and $2+5i$. Adding $(3-4i)+(2+5i)$, we add the real parts and the imaginary parts.

$3+2-4i+5i$

$5+i$

Try It

Subtract $2+5i$ from $3-4i$.

Answer: $(3-4i)-(2+5i)=1-9i$

[ohm_question]61710[/ohm_question]
In the following video, we present more worked examples of arithmetic with complex numbers. https://youtu.be/XJXDcybM84U When we add complex numbers, we can visualize the addition as a shift, or translation, of a point in the complex plane.

Example

Visualize the addition $3-4i$ and $-1+5i$. The initial point is $3-4i$. When we add $-1+3i$, we add $-1$ to the real part, moving the point 1 units to the left, and we add 5 to the imaginary part, moving the point 5 units vertically. This shifts the point $3-4i$ to $2+1i$.

Try It

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We can also multiply complex numbers by a real number, or multiply two complex numbers.

Example

Multiply: $4\left(2+5i\right)$ To multiply the complex number by a real number, we simply distribute as we would when multiplying polynomials. Distribute and simplify.

$4(2+5i)\\\,\,\,= 4\cdot2+4\cdot5i\\\,\,\,=8+20i$

Example

Multiply: $(2+5i)(4+i)$.

$\begin{array}{l}\left(2+5i\right)\left(4+i\right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{Expand.}\\=8+20i+2i+5i^{2}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{Since }i=\sqrt{-1},i^{2}=-1\\=8+20i+2i+5\left(-1\right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{Simplify.}\\=3+22i\end{array}$

Try It

Multiply $3-4i$ and $2+3i$.

Answer: Multiply $(3-4i)(2+3i)=6+9i-8i-12{{i}^{2}}=6+i-12(-1)=18+i$

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To understand the effect of multiplication visually, we’ll explore three examples.

Example

Visualize the product $2(1+2i)$. Multiplying we’d get

\begin{align}&2\cdot1+2\cdot2i\\&=2+4i\\\end{align}

Notice both the real and imaginary parts have been scaled by 2. Visually, this will stretch the point outwards, away from the origin.

Example

Visualize the product $i\left(l+2i\right)$. Multiplying, we’d get

\begin{align}&i\cdot1+i\cdot2i\\&=i+2{{i}^{2}}\\&=i+2(-1)\\&=-2+i\\\end{align}

In this case, the distance from the origin has not changed, but the point has been rotated about the origin, 90° counter-clockwise.

Try It

Multiply $3-4i$ and $2+3i$.

Answer: Multiply $(3-4i)(2+3i)=6+9i-8i-12{{i}^{2}}=6+i-12(-1)=18+i$

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Example

Visualize the result of multiplying $1+2i$ by $1+i$. Then show the result of multiplying by $1+i$ again. Multiplying $1+2i$ by $1+i$,

$-4+2i$

\begin{align}&(1+2i)(1+i)\\&=1+i+2i+2{{i}^{2}}\\&=1+3i+2(-1)\\&=-1+3i\\\end{align}

Multiplying by $1+i$ again,

\begin{align}&(-1+3i)(1+i)\\&=-1-i+3i+3{{i}^{2}}\\&=-1+2i+3(-1)\\&=-4+2i\\\end{align}

If we multiplied by $1+i$ again, we’d get $–6–2i$. Plotting these numbers in the complex plane, you may notice that each point gets both further from the origin, and rotates counterclockwise, in this case by 45°.
In general, multiplication by a complex number can be thought of as a scaling, changing the distance from the origin, combined with a rotation about the origin.

Try It

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The following video presents more examples of how to visualize the results of arithmetic on complex numbers. In the following video, we present more worked examples of arithmetic with complex numbers. https://youtu.be/vPZAW7Lhh1E