3.7 Use the Complex Number System

Evaluate the Square Root of a Negative Number

Whenever we have a situation where we have a square root of a negative number we say there is no real number that equals that square root. For example, to simplify [latex]\sqrt{-1}[/latex], we are looking for a real number [latex]x[/latex] so that [latex]x^2=–1[/latex]. Since all real numbers squared are positive numbers, there is no real number that equals [latex]–1[/latex] when squared.

Mathematicians have often expanded their numbers systems as needed. They added [latex]0[/latex] to the counting numbers to get the whole numbers. When they needed negative balances, they added negative numbers to get the integers. When they needed the idea of parts of a whole they added fractions and got the rational numbers. Adding the irrational numbers allowed numbers like [latex]\sqrt{5}[/latex] All of these together gave us the real numbers and so far in your study of mathematics, that has been sufficient.

But now we will expand the real numbers to include the square roots of negative numbers. We start by defining the imaginary unit [latex]i[/latex] as the number whose square is [latex]–1[/latex].

We will use the imaginary unit to simplify the square roots of negative numbers.

Square Root of a Negative Number

We will use this definition in the next example. Be careful that it is clear that the [latex]i[/latex] is not under the radical. Sometimes you will see this written as [latex]\sqrt{-b}=i\sqrt{b}[/latex] to emphasize the [latex]i[/latex] is not under the radical. But the [latex]\sqrt{-b}=\sqrt{b}\;i[/latex] is considered standard form.

Now that we are familiar with the imaginary number [latex]i[/latex], we can expand the real numbers to include imaginary numbers. The complex number system includes the real numbers and the imaginary numbers. A complex number is of the form [latex]a+bi[/latex], where [latex]a[/latex], [latex]b[/latex] are real numbers. We call [latex]a[/latex] the real part and [latex]b[/latex] the imaginary part.

If [latex]b=0[/latex], then [latex]a+bi[/latex] becomes [latex]a+0\times i=a[/latex], and is a real number.

If [latex]b\ne 0[/latex], then [latex]a+bi[/latex] is an imaginary number.

If [latex]a=0[/latex], then [latex]a+bi[/latex] becomes [latex]0+bi=bi[/latex], and is called a pure imaginary number.

We summarize this here.

The standard form of a complex number is [latex]a+bi[/latex], so this explains why the preferred form is [latex]\sqrt{\text{−}b}=\sqrt{b}i[/latex] when [latex]b >0[/latex]

The diagram helps us visualize the complex number system. It is made up of both the real numbers and the imaginary numbers.

[latex]\begin{array}{ccc}\xrightarrow[\boxed{\begin{array}{c}\text{Real Numbers}\\a+bi\;\;\;\;b=0\end{array}}]{}&\boxed{\begin{array}{c}\text{Complex Numbers}\\a+bi\end{array}}&\xleftarrow[\boxed{\begin{array}{c}\text{Imaginary Numbers}\\a+bi\;\;\;\;b\neq 0\end{array}}]{}\end{array}[/latex]

Add or Subtract Complex Numbers

We are now ready to perform the operations of addition, subtraction, multiplication and division on the complex numbers—just as we did with the real numbers.

Adding and subtracting complex numbers is much like adding or subtracting like terms. We add or subtract the real parts and then add or subtract the imaginary parts. Our final result should be in standard form.

Remember to add both the real parts and the imaginary parts in this next example.

Multiply Complex Numbers

Multiplying complex numbers is also much like multiplying expressions with coefficients and variables. There is only one special case we need to consider. We will look at that after we practice in the next two examples.

In the next example, we multiply the binomials using the Distributive Property or FOIL.

In the next example, we could use FOIL or the Product of Binomial Squares Pattern.

Since the square root of a negative number is not a real number, we cannot use the Product Property for Radicals. In order to multiply square roots of negative numbers we should first write them as complex numbers, using [latex]\sqrt{-b}=\sqrt{b}i[/latex] This is one place students tend to make errors, so be careful when you see multiplying with a negative square root.

In the next example, each binomial has a square root of a negative number. Before multiplying, each square root of a negative number must be written as a complex number.

We first looked at conjugate pairs when we studied polynomials. We said that a pair of binomials that each have the same first term and the same last term, but one is a sum and one is a difference is called a conjugate pair and is of the form [latex]\left(a-b\right),\left(a+b\right)[/latex]

A complex conjugate pair is very similar. For a complex number of the form [latex]a+bi[/latex], its conjugate is [latex]a-bi[/latex]. Notice they have the same first term and the same last term, but one is a sum and one is a difference.

Complex Conjugate Pair

We will multiply a complex conjugate pair in the next example.

From our study of polynomials, we know the product of conjugates is always of the form [latex]\left(a-b\right)\left(a+b\right)={a}^{2}-{b}^{2}[/latex]. The result is called a difference of squares. We can multiply a complex conjugate pair using this pattern.

The last example we used FOIL. Now we will use the Product of Conjugates Pattern.

[latex]\begin{align*}&\left(\overset{{\color{red}{a}}{\color{red}{-}}{\color{red}{b}}}{3-2i}\right)\left(\overset{{\color{red}{a}}{\color{red}{+}}{\color{red}{b}}}{3+2i}\right)\\&=\overset{{\color{red}{a}}^{\color{red}{2}}}{\left(3\right)^2}\overset{\color{red}{-}}-\overset{{\color{red}{b}}^{\color{red}{2}}}{\left(2i\right)^2}\\&=9-4i^2\\&=9-4(-1)\\&=13\end{align*}[/latex]

When we multiply complex conjugates, the product of the last terms will always have an [latex]{i}^{2}[/latex] which simplifies to [latex]-1[/latex]

This leads us to the Product of Complex Conjugates Pattern: [latex]\left(a-bi\right)\left(a+bi\right)={a}^{2}+{b}^{2}[/latex]

Product of Complex Conjugates

Dividing complex numbers is much like rationalizing a denominator. We want our result to be in standard form with no imaginary numbers in the denominator.

We summarize the steps here.

The powers of [latex]i[/latex] make an interesting pattern that will help us simplify higher powers of [latex]i[/latex]. Let’s evaluate the powers of [latex]i[/latex] to see the pattern.

We summarize this now.

If we continued, the pattern would keep repeating in blocks of four. We can use this pattern to help us simplify powers of [latex]i[/latex]. Since [latex]i^4=1[/latex], we rewrite each power, [latex]i^n[/latex], as a product using [latex]i^4[/latex] to a power and another power of [latex]i[/latex].

We rewrite it in the form [latex]{i}^{n}={\left({i}^{4}\right)}^{q}·{i}^{r}[/latex], where the exponent, [latex]q[/latex], is the quotient of [latex]n[/latex] divided by [latex]4[/latex] and the exponent, [latex]r[/latex], is the remainder from this division. For example, to simplify [latex]i^{57}[/latex], we divide [latex]57[/latex] by [latex]4[/latex] and we get [latex]14[/latex] with a remainder of [latex]1[/latex]. In other words, [latex]57=4·14+1[/latex] So we write [latex]{i}^{57}={\left({1}^{4}\right)}^{14}·{i}^{1}[/latex] and then simplify from there.