Plotting points was something you did back in early algebra, and it likely seems pretty simple now. For instance, plotting the point (4, 5) meant starting at the origin and moving 4 units to the right - the *x* direction, and 5 units up - the *y* direction.

In this lesson, one of the things we will consider is the graphing of complex numbers such as \begin{align*}4 +3i\end{align*}

In essence it doesn't sound that hard, but which direction would you move 3 imaginary units?

### Complex Numbers

Sometimes when you solve a quadratic equation, the solution has both a real part and an imaginary part. For example, if you want to solve

\begin{align*}(x - 1)^2 + 4 = 0\end{align*}

then

\begin{align*}(x - 1)^2 = -4\end{align*}

\begin{align*}x - 1 = \pm\sqrt{-4}\end{align*}

\begin{align*}x - 1 = \pm\sqrt{-1}\sqrt{4}\end{align*}

\begin{align*}x - 1 = \pm2i\end{align*}

\begin{align*}x = 1 \pm 2i\end{align*}

\begin{align*}x = 1 + 2i\end{align*}*or* \begin{align*}1 - 2i\end{align*}

Notice that these two solutions involve a real part, 1, and an imaginary part, ±2*i*

*a* + *bi* is the **standard** or **rectangular** form of a complex number.

A complex number is a number that has a real part (in this case *a*) and an imaginary part, that is, the imaginary number *i* with a coefficient *b*.

The complex numbers are a superset of the real numbers, meaning that all of the real numbers are part of the set of complex numbers:

Given *a* + *bi*, if *b* = 0 (meaning there is no imaginary part to the complex number), then all you have remaining is a real number. Viewed this way, every real number can be written as a complex number (just let it equal *a*), but there are many more complex numbers than real numbers. Hence the complex numbers are a superset of the real numbers.

#### Graphing Complex Numbers

In standard form, *a* + *bi*, a complex number can be graphed using rectangular coordinates (*a*, *b*). The *x* - coordinate represents “real number” values, while the *y* - coordinate represents the “imaginary” values.

### Examples

#### Example 1

Earlier, you were asked a question about which direction you move when you graph imaginary numbers.

You of course don't only *imagine* moving in one direction!

When graphing complex numbers in standard form: *a* + *bi*, the real component *a* is plotted on the horizontal or *x* axis. The imaginary component *bi* is plotted on the *y* axis in just the same way a real number would be.

\begin{align*}\therefore 4 + 3i\end{align*}

#### Example 2

Graph the complex number: *z* = 2 + 2*i*, in rectangular form.

The coordinate (2, 2) is graphed as shown below:

#### Example 3

Solve each equation and express it as a complex number. (Note: If the imaginary part is 0, express the solution as *a* + *0i*)

*x*^{2}+ 24 = 0

\begin{align*}x = \pm2\sqrt{6}i\end{align*}

- 2
*x*^{2}- 4*x*+ 7 = 0

\begin{align*}x = 1 \pm 2\sqrt{3}i\end{align*}

#### Example 4

Plot each of the following complex numbers in rectangular form:

- (4 + 2
*i*) - (-3 +
*i*) - (3 - 4
*i*) - 3
*i*

Your graph should look like:

#### Example 5

Simplify and express as a complex number \begin{align*}-\sqrt{60} + \sqrt{-121}\end{align*}

\begin{align*}-\sqrt{4 \cdot 15} + \sqrt{121 \cdot -1}\end{align*}

\begin{align*}-2\sqrt{15} + 11\sqrt{-1}\end{align*}

\begin{align*}-2\sqrt{15} + 11i\end{align*}

#### Example 6

Solve the equation and express the answer as a simplified complex number \begin{align*}x(4x) + 4 = 0\end{align*}

\begin{align*}4x^2 + 4 = 0\end{align*}*x*

\begin{align*}4x^2 = -4\end{align*}

\begin{align*}x^2 = -1\end{align*}

\begin{align*}x = i\end{align*}

#### Example 7

Graph the complex numbers:

- 3 + 2i
- 2 - 3i
- -2 + 2i

To graph the complex numbers, graph the real value on the *x* axis, and the imaginary value on the *y* axis. The image below shows the points correctly graphed.

### Review

- What are complex numbers technically the sum of?
- What does the complex plane represent?

Express in simplest form in terms of \begin{align*}i\end{align*}

- \begin{align*}13 - \sqrt{-49}\end{align*}
13−−49−−−−√ - \begin{align*}\sqrt{75} + \sqrt{\frac{-16}{256}}\end{align*}
75−−√+−16256−−−−√ - \begin{align*}-3 - \sqrt{\frac{-25}{169}}\end{align*}
−3−−25169−−−−√ - \begin{align*}-\sqrt{36} + \sqrt{-64}\end{align*}
−36−−√+−64−−−−√ - \begin{align*}10 - \sqrt{\frac{-4}{36}}\end{align*}
10−−436−−−√ - \begin{align*}4 + \sqrt{-250}\end{align*}
4+−250−−−−√ - \begin{align*}3^2 \sqrt{-0.0009}\end{align*}
32−0.0009−−−−−−−√ - \begin{align*}\sqrt{-0.16} -(- \sqrt{27})\end{align*}
−0.16−−−−−√−(−27−−√) - \begin{align*}10 + \sqrt{-0.49}\end{align*}
10+−0.49−−−−−√ - \begin{align*}-3 + \sqrt{-0.0196}\end{align*}
−3+−0.0196−−−−−−−√ - \begin{align*}9 \sqrt{-8j^9} + 3\sqrt{25}\end{align*}
9−8j9−−−−√+325−−√ - \begin{align*}\sqrt{-676b^3a^1c^8} +\sqrt{112}\end{align*}
−676b3a1c8−−−−−−−−−√+112−−−√

- Two complex numbers are graphed below. What are the numbers expressed in standard complex number form?

- Graph the complex number \begin{align*}3 - 4i\end{align*}.
- Graph the complex number \begin{align*}-2 + 3i\end{align*}.
- Graph the complex point \begin{align*}(3 + i)\end{align*}.
- Graph the complex point \begin{align*}(-1 - 2i)\end{align*}.

### Review (Answers)

To see the Review answers, open this PDF file and look for section 4.5.