www.ck12.org Chapter 2. Radical Equations and Radical Functions

2.7 Imaginary and Complex Numbers

Learning Objectives

• Write square roots with negative radicands in terms of i • Recognize and write complex numbers in standard form • Describe the relationship between the sets of integers, rational numbers, real numbers and complex numbers • Plot z = a + bi in the complex number plane

Introduction

When working with quadratic equations, some quadratic equations have solutions that are not real. For example, an equation such as:

(x − 1)2 + 4 = 0 does not have real solutions. No matter which method of solving quadratics we use, the solutions to that equation are not real numbers.

(x − 1)2 + 4 = 0 (x − 1)2 = −4  √ (x − 1)=± −4 √ x − 1 = ± −4 √ x = 1 ± −4 x = 1 ± 2i √ √ Notice how the −4 simplified to 2i. This is because −1 = i. The solutions 1 + 2 i and 1 - 2i combine imaginary and real numbers, and are called complex numbers. The use of the word imaginary does not mean these numbers are useless. For a long period in the history of mathematics, it was thought that the square root of a negative number was in fact only within the mathematical imagination without real-world significance hence, imaginary. That has changed. Mathematicians now consider the imaginary number as another set of numbers that have real significance, but do not fit on what is called the number lineengineers, scientists, and others solve real world problems using complex numbers! √ Where did complex numbers come from? If you solve the equation x2 + 1 = 0, you get x = ± −1. But there is no real number that, when multiplied by itself, yields -1. To fix this problem, mathematicians defined the imaginary constant, i. by definition,

√ i = −1

or squaring both sides,

119 2.7. Imaginary and Complex Numbers www.ck12.org

i2 = −1 √ √ √Recall√ that you√ can simplify radicals by factoring out perfect squares in the radicand. For instance, 8 = 4 · 2 = 4 2 = 2 2. The same procedure√ works with i. If you have a negative number in the radicand, you can factor out the -1 and use the identity i = −1 to simplify. √ Example: Simplify −5 Solution: √  −5 = (−1) · (5) √ √ = −1 5 √ √ = i 5or 5i This also works in combination with the other method of factoring out perfect squares. See the following example. Notice the i can go before the square root or after the square root. However, the i can not go under the square root. √ Example: Simplify −72 Solution: √  −72 = (−1) · (72) √ √ = −1 72 √ = i 72 But, we’re not done yet. 72 = 36·2, so √ √ √ i 72 = i 36 2 √ = i(6) 2 √ √ = 6i 2or6 2i

Standard Form of 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

(x + 2)2 + 8 = 0 then

(x + 2)2 = −8

√ x + 2 = ± −8

√ √ x + 2 = ± −1 2 · 2 · 2

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√ x + 2 = ±2i 2

√ x = −2 ± 2i 2

√ √ x = −2 + 2i 2orx = −2 − 2i 2

√ Notice that these two solutions involve a real part, -2, and an imaginary part, ±2 2i z = 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.

Set of Complex Numbers (complex, real, irrational, rational, etc.)

The complex numbers are a superset of the real numbers. Given z = a + bi,ifb = 0 then z is a real number. Every real number can be written as a complex number (just let it equal abwith b=0), but there are many more complex numbers than real numbers. Hence the complex numbers are a superset of the real numbers. When you were first introduced to mathematics, you probably begab by counting positive whole numbers, that is 1, 2, 3, 4,... Later, negative whole numbers were investigated. The set of all whole numbers, both positive and negative, including the number zero, is known as integers: ... - 2, - 1, 0, 1, 2,... Later, students were introduced to fractions. The set of all numbers that CAN be expressed as a quotient of two integers (where the denominator is not zero) is called the set of Rational Numbers. Rational Numbers can also − , 3 ,− 7 , . ,− . , be expressed as a terminating or repeating decimal. Some rational numbers are 1 5 3 0 6 5 27 0. Of course there are an infinite number of rational numbers between any two whole numbers, so listing all rational numbers neatly is difficult (but it is possible–can you think of a way to do it?). Notice, that all integers are in the set of rational numbers (for example, 5 CAN be written as the quotient of 10 and = 10 2 since 5 2 ), so the integers are a subset of the rational numbers. Finally, when working with circles students encounter a number that can be approximated as a quotient of two integers but cannot be expressed EXACTLY as π π 22 that quotient, that is the number . Recall that was often expressed as APPROXIMATELY 7 or 3.14, BUT NOT EXACTLY THOSE VALUES. When first√ exploring using the√ Pythagorean Theorem to find the length of a diagonal of a square whose side is 1, the number 2 was introduced.√ 2 often was approximated as 1.4 or 1.414, but again you can’t possibly write out all of the decimals in 2. These two numbers are examples of IRRATIONAL numbers, that is numbers that cannot be expressed as a quotient of two integers, and therefore CANNOT be expressed as a terminating or repeating decimal. The set of all rational and irrational numbers together is called REAL numbers. The set of all real numbers AND imaginary numbers is called the set of Complex numbers.

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Complex Number Plane

In standard form z = a + bi, a complex number can be graphed using rectangular coordinates (a, b). a represents the x - coordinate, while b represents the y - coordinate. Alternatively, the x - coordinate represents “real number” values, while the y - coordinate represents the “imaginary” values. For example, given the complex number in standard form: z =2+2i, you can graph this number in the coordinate plane To graph this point, the coordinate (2, 2) is graphed as shown below:

Lesson Summary

When graphing a complex number using rectangular coordinates, the x-axis plots the real number, while the y-axis plots the coefficient of the imaginary number.

Review Questions

Using rectangular coordinate system, graph

1. Simplify the following radicals

122 www.ck12.org Chapter 2. Radical Equations and Radical Functions √ a. √−9 b. √−12 c. √−17 d. 140 − 108 2. Solve each equation and express it as a complex number. (Note: If the imaginary part is 0, you can still express the solution as a + bi, but you will have b=0 a. x2 +24=0 b. 2x2 -4x +7=0 3. Plot each of the following complex numbers: a. (4 + 2i) b. (-3 + i) c. (3 - 4i) d. 3i

Review Answers

1. a. 3i √ b. 2√i 3 c. i√ 17 √ d. 32 =√4 2 2. a. x = ±2 √6i √ = ± 10i 2± 10i b. x 1 2 or 2

3.

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