Lesson Objectives
  • Demonstrate an understanding of the distance formula
  • Learn how to find the absolute value of a complex number

How to Find the Absolute Value of a Complex Number


In this lesson, we will learn how to find the absolute value of a complex number. At this point, we should fully understand the concept of absolute value with real numbers. We know the absolute value of a number such as 5 is 5 because the number 5 is 5 units away from zero on the number line. Starting at 5, we can count 5 units to get to zero. showing a number line When we talk about the absolute value of a complex number, we use the same concept. The absolute value of a complex number is also a measure of its distance from zero. The only difference is we are measuring the distance on the complex plane.
Example #1: Find the absolute value of the given complex number. $$|5 + 7i|$$ Let’s begin by plotting this complex number 5 + 7i on the complex plane. We know we would move five units right on the real axis and 7 units up on the imaginary axis. graphing 5 + 7i on the complex plane When we ask for the absolute value of a complex number, also known as the modulus, we are asking for the distance from the origin to the complex number on the complex plane. graphing 5 + 7i on the complex plane We are really familiar with our Pythagorean formula at this point and we know we can use it to find the distance here. Let’s create a right triangle. To do this, we will have a point at the origin, a point at 5 + 7i, and a point at 5 + 0i. graphing a right triangle on the complex plane So what’s the measure of the vertical leg here? It’s the distance from 7, the value on the imaginary axis to the origin, where the value is 0 on the imaginary axis. This distance will be 7. What's the measure of the horizontal leg here? It’s the distance from 5 on the real axis to the origin, where the value is 0 on the real axis. The distance will be 5. At this point, we know that: $$a^2 + b^2=c^2$$ $$a=5, b=7$$ $$5^2 + 7^2=c^2$$ $$25 + 49=c^2$$ $$c^2=74$$ $$c=\sqrt{74}$$ This tells us the absolute value of 5 + 7i is the square root of 74. $$|5 + 7i|=\sqrt{74}$$ Let's now consider a shortcut to this process. Since one point on our right triangle will always be the origin: 0 + 0i, our vertical leg will be |b - 0| or just |b| and our horizontal leg will be |a - 0| or just |a|.
Note: a and b here refer to the real part (a) and the imaginary part (b) of the complex number. $$|a|^2 + |b|^2=c^2$$ We can drop the absolute value bars since squaring makes our answer non-negative. We end up with: $$a^2 + b^2=c^2$$ $$c=\sqrt{a^2 + b^2}$$ Since c is the absolute value of our complex number, we can replace it: $$|a + bi|=\sqrt{a^2 + b^2}$$ If we repeat our problem with this simpler approach, we get: $$|5 + 7i|$$ $$a=5$$ $$b=7$$ $$|5 + 7i|=\sqrt{5^2 + 7^2}$$ $$|5 + 7i|=\sqrt{25 + 49}$$ $$|5 + 7i|=\sqrt{74}$$

Skills Check:

Example #1

Find the absolute value. $$4 - 2i$$

Please choose the best answer.

A
$$2\sqrt{5}$$
B
$$2\sqrt{3}$$
C
$$5$$
D
$$\sqrt{17}$$
E
$$2\sqrt{13}$$

Example #2

Find the absolute value. $$3 - 2i$$

Please choose the best answer.

A
$$\sqrt{26}$$
B
$$5$$
C
$$\sqrt{5}$$
D
$$\sqrt{13}$$
E
$$2\sqrt{13}$$

Example #3

Find the absolute value. $$-3\sqrt{2}+ 3i\sqrt{2}$$

Please choose the best answer.

A
$$\sqrt{5}$$
B
$$2$$
C
$$\sqrt{10}$$
D
$$6$$
E
$$3$$
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