Topics
3. Quadratic Equations
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United States of America
Grades 9-12

3.08 Operations with complex numbers

Lesson
Practice

Introduction

In Math 2 lesson  1.01 The real number system  , we reviewed the number types we had learned throughout elementary and middle school. In this lesson, we will explore two new types of numbers: imaginary numbers and complex numbers.

Imaginary numbers

We have found that quadratic equations can have non-real solutions, when the discriminant is less than zero. To define these non-real solutions, we use imaginary numbers which are built on the concept that there is a number, called i such that i^2=-1.

Imaginary number

A number that, when squared, results in a negative real number. It can be expressed as a real multiple of i, defined by i = \sqrt{-1}.

This is not a real number, since the square of any real number is always non-negative. However, using this new number, we can find the answers to many real-world problems in electrical engineering, quantum physics, and more.

Note that the words “real” and “imaginary” are just names for different kinds of numbers. It does not mean that one type of number exists while the other type does not exist.

We can express the square root of any negative number by taking out a factor of \sqrt{-1}=i to get a imaginary number. For example:\sqrt{-5}=\sqrt{-1\cdot 5}=\sqrt{-1}\sqrt{5}=i\sqrt{5} \text{ or }\sqrt{5}i

Exploration

Calculate the powers of i, from i^2 until i^8.

  1. What pattern do you notice?
  2. How can you use the pattern to find i^{23},\,i^{30},\,i^{33}, and i^{40}?

When we raise i to some power, we notice a pattern unfolds:

i^1=ii^5=ii^9=i
i^2=-1i^6=-1i^{10}=-1
i^3=-ii^7=-ii^{11}=-i
i^4=1i^8=1i^{12}=1

We can see that there is a cycle of 4, and we can use this pattern to determine the value of any power of i.

When the exponent of i is a multiple of 4, the expression simplifies to 1. When the exponent is one more than a multiple of 4, the expression simplifies to i. For exponents that are 2 more or 3 more than a multiple of 4, the expression simplifies to -1 or -i respectively. We can summarize this algebraically like so:

i^{4n+1}=i
i^{4n+2}=-1
i^{4n+3}=-i
i^{4n}=1

This shows that we can write any exponent in the form {4n+r} where n is any integer and r can be 0, 1, 2, or 3.

Examples

Example 1

Express the following in terms of i:

a

\sqrt{-49}

Worked Solution
Create a strategy

If we use the multiplication property of radicals, we can separate the radicand into -1\cdot 49 to define with i.

Apply the idea
\displaystyle \sqrt{-49}\displaystyle =\displaystyle \sqrt{-1\cdot 49}Factor out -1
\displaystyle =\displaystyle \sqrt{-1}\cdot\sqrt{49}Product of square roots
\displaystyle =\displaystyle i\cdot 7Definition of i
\displaystyle =\displaystyle 7iCommutative property of multiplication
b

\sqrt{-10}

Worked Solution
Create a strategy

We will use the same reasoning as the example above.

Apply the idea
\displaystyle \sqrt{-10}\displaystyle =\displaystyle \sqrt{-1\cdot 10}Factor out -1
\displaystyle =\displaystyle \sqrt{-1}\cdot\sqrt{10}Product of square roots
\displaystyle =\displaystyle i\sqrt{10}Definition of i

Since the radical cannot be simplified further, this is the final answer.

Reflect and check

This answer can also be written as \sqrt{10}i, but it needs to be clear that the i is not underneath the radical.

c

\sqrt{-32}

Worked Solution
Create a strategy

After writing the expression in terms of i, this radical can be simplified which we can do using properties of radicals. Our goal is to find the largest perfect square factor of 32.

Apply the idea
\displaystyle \sqrt{-32}\displaystyle =\displaystyle \sqrt{-1\cdot 32}Factor out -1
\displaystyle =\displaystyle \sqrt{-1}\cdot \sqrt{32}Product of radicals
\displaystyle =\displaystyle i\sqrt{32}Substitute i=\sqrt{-1}
\displaystyle =\displaystyle i\sqrt{16\cdot 2}Factor 32
\displaystyle =\displaystyle i\sqrt{16}\sqrt{2}Product of radicals
\displaystyle =\displaystyle 4i\sqrt{2}Evaluate the radical
Reflect and check

As you become more familiar with imaginary numbers, you can use fewer steps to simplify:

\displaystyle \sqrt{-32}\displaystyle =\displaystyle \sqrt{-16\cdot 2}
\displaystyle =\displaystyle \sqrt{-16}\cdot\sqrt{2}
\displaystyle =\displaystyle 4i\sqrt{2}
Idea summary

Imaginary numbers are used to define square roots of negative numbers. The powers of i follow a cyclic pattern of 4:

i^1i^2i^3i^4\cdots
i-1-i1\cdots

Complex numbers

Previously, all the numbers we knew fell under the umbrella of real numbers. Now, we have learned about imaginary numbers which are not real numbers, so we need to introduce a new type of number that encompasses both real and imaginary numbers.

Complex number

A number that can be written in the form a+bi, where a and b are real numbers and i is defined as the imaginary number where i^2 = -1.

By this definition, all numbers are complex numbers. Sometimes, a is referred to as the real part and bi is called the imaginary part. Both parts together make up one complex number.

A real number is a complex number where b=0. For example, -5=-5+0i.

A pure imaginary number is a complex number where a=0. For example, 2i=0+2i.

A diagram showing the Complex Number System. Under the complex number system is the Real Number System, and numbers like square root of negative 2 and i. The Real Number System is divided into Rational and Irrational. Within Rational is the subset Integers; within Integers is the subset Whole; within Whole is the subset Natural. Examples of each type of number are shown. Speak to your teacher for more information.

We perform the algebraic operations for complex numbers the same way we perform operations for rational algebraic expressions, except we sometimes have an extra step to account for the powers of i.

Addition: add in the same way as binomials with like terms

General example\left(a+bi\right)+\left(c+di\right)=\left(a+c\right)+\left(b+d\right)i
Numerical example\left(2+3i\right)+\left(4+5i\right)=\left(2+4\right)+\left(3+5\right)i=6+8i

Subtraction: subtract in the same way as binomials with like terms

General example\left(a+bi\right)-\left(c+di\right)=\left(a-c\right)+\left(b-d\right)i
Numerical example(2+3i)-(4+5i)=(2-4)+(3-5)i=-2-2i

Multiplication: distribute in the same way as binomials, evaluate any powers of i, then combine any like terms

General example\left(a+bi\right)\left(c+di\right)=ac+adi+bci+bdi^2=ac+\left(ad+bc\right)i+bdi^2=\left(ac-bd\right)+\left(ad+bc\right)i
Numerical example(2+3i)(4+5i)=8+10i+12i+15i^2=8+(10+12)i+15i^2={(8-15)+(10+12)i}=-7+22i

Notice the commutative, associative, and distributive properties were applied to perform these operations to complex numbers. Also notice that when we add, subtract, and multiply complex numbers, the answer is in the form of another complex number, a+bi. In other words, complex numbers are closed under addition, subtraction, and multiplication.

Examples

Example 2

The following complex numbers are written in the form a+bi. State the values a and b.

a

-8+i

Worked Solution
Apply the idea

a=-8

b=1

Reflect and check

Both terms together are considered one complex number.

b

\dfrac{4}{7}

Worked Solution
Create a strategy

This is a real number which means the imaginary part is missing, so we can write the imaginary part as 0i. That means \dfrac{4}{7} can be written as \dfrac{4}{7}+0i.

Apply the idea

a=\dfrac{4}{7} and b=0

Reflect and check

\dfrac{4}{7} can be classified as complex, real, and rational.

c

\sqrt{-40}

Worked Solution
Create a strategy

We need to rewrite the expression in terms of i, simplify the radical, then determine the values of a and b.

Apply the idea
\displaystyle \sqrt{-40}\displaystyle =\displaystyle \sqrt{-4\cdot 10}Factor out -4
\displaystyle =\displaystyle \sqrt{-4}\sqrt{10}Product of square roots
\displaystyle =\displaystyle 2i\sqrt{10}Evaluate the square root

There is only one term in this expression, and it contains i. This means the real part is missing which makes this a pure imaginary number.

2i\sqrt{10}=0+2\sqrt{10}i

a=0

b=2\sqrt{10}

Reflect and check

This number can be classified as complex, imaginary, and pure imaginary.

Example 3

Simplify each of the following expressions. Justify each step using the commutative, associative, and distributive properties.

a

\left( -3 + 5 i\right) + \left(7-2 i\right)

Worked Solution
Create a strategy

When adding or subtracting complex numbers, we can combine like terms by adding the real parts together and the imaginary parts together.

Apply the idea
\displaystyle \left( -3 + 5 i\right) + \left(7-2 i\right)\displaystyle =\displaystyle (-3 +7)+ (5 i -2 i)Commutative and associative properties
\displaystyle =\displaystyle 4+3iEvaluate the addition
b

(-6-i)-(8-5i)

Worked Solution
Apply the idea
\displaystyle (-6-i)-(8-5i)\displaystyle =\displaystyle (-6-i)+(-8+5i)Distributive property
\displaystyle =\displaystyle (-6+-8)+(-i+5i)Commutative and associative properties
\displaystyle =\displaystyle -14+4iEvaluate the addition
Reflect and check

It is important to distribute the negative sign to the second complex number first because the commutative and associative properties only hold for addition and multiplication, not subtraction.

c

\left( 2 - 4 i\right) \left(-4+2 i\right)

Worked Solution
Create a strategy

When multiplying complex numbers, we can use the distributive property to distribute the parentheses: \left(A+B\right)\left(C+D\right)=AC+AD+BC+BD

Apply the idea
\displaystyle \left( 2 - 4 i\right) \left(-4+2 i\right)\displaystyle =\displaystyle 2\left(-4\right)+2\left(2i\right)-4i\left(-4\right)-4i\left(2i\right)Distributive property
\displaystyle =\displaystyle -8+4i+16i-8i^2Evaluate the multiplication
\displaystyle =\displaystyle -8+4i+16i-8(-1)Substitute i^2=-1
\displaystyle =\displaystyle -8+4i+16i+8Evaluate the multiplication
\displaystyle =\displaystyle 20iEvaluate the addition

Example 4

Rewrite x^{2} + 16 in form (a+bi)(c-di).

Worked Solution
Create a strategy

We will use the identity a^2+b^2=\left(a+bi\right)\left(a-bi\right) to rewrite this expression.

Apply the idea

First find the square root of each term.

\sqrt{x^2}=x and \sqrt{16}=4.

Then substitute a=x and b=4 into the identity a^2+b^2=\left(a+bi\right)\left(a-bi\right).

x^{2} + 16=(x+4i)(x-4i).

Idea summary

All numbers are complex numbers. They are in the form a+bi where a is the real part and bi is the imaginary part. Both parts together make one complex number.

We simplify complex numbers by combining the real parts with the imaginary parts. Sometimes, we have to evaluate powers of i and continue simplifying the expression.

Outcomes

N.CN.A.1

Know there is a complex number I such that i^2 = -1, and every complex number has the form a + bi with a and b real.

N.CN.A.2

Use the relation i^2 = -1 and the commutative, associative, and distributive properties to add, subtract, and multiply complex numbers.

N.CN.C.8 (+)

Extend polynomial identities to the complex numbers.

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