## Section 6.5: Polar Form of Complex Numbers

### Learning Outcomes

• Plot complex numbers in the complex plane.
• Find the absolute value of a complex number.
• Write complex numbers in polar form.
• Convert a complex number from polar to rectangular form.
• Find products of complex numbers in polar form.
• Find quotients of complex numbers in polar form.
• Find powers and roots of complex numbers in polar form.

“God made the integers; all else is the work of man.” This rather famous quote by nineteenth-century German mathematician Leopold Kronecker sets the stage for this section on the polar form of a complex number. Complex numbers were invented by people and represent over a thousand years of continuous investigation and struggle by mathematicians such as Pythagoras, Descartes, De Moivre, Euler, Gauss, and others. Complex numbers answered questions that for centuries had puzzled the greatest minds in science.

We first encountered complex numbers in Precalculus I. In this section, we will focus on the mechanics of working with complex numbers: translation of complex numbers from polar form to rectangular form and vice versa, interpretation of complex numbers in the scheme of applications, and application of De Moivre’s Theorem.

## Plotting Complex Numbers in the Complex Plane

Plotting a complex number $a+bi$ is similar to plotting a real number, except that the horizontal axis represents the real part of the number, $a$, and the vertical axis represents the imaginary part of the number, $bi$.

### How To: Given a complex number $a+bi$, plot it in the complex plane.

1. Label the horizontal axis as the real axis and the vertical axis as the imaginary axis.
2. Plot the point in the complex plane by moving $a$ units in the horizontal direction and $b$ units in the vertical direction.

### Example 1: Plotting a Complex Number in the Complex Plane

Plot the complex number $2 - 3i$ in the complex plane.

### Try It

Plot the point $1+5i$ in the complex plane.

## Finding the Absolute Value of a Complex Number

The first step toward working with a complex number in polar form is to find the absolute value. The absolute value of a complex number is the same as its magnitude, or $|z|$. It measures the distance from the origin to a point in the plane. For example, the graph of $z=2+4i$, in Figure 2, shows $|z|$.

Figure 2

### A General Note: Absolute Value of a Complex Number

Given $z=x+yi$, a complex number, the absolute value of $z$ is defined as

$|z|=\sqrt{{x}^{2}+{y}^{2}}$

It is the distance from the origin to the point $\left(x,y\right)$.

Notice that the absolute value of a real number gives the distance of the number from 0, while the absolute value of a complex number gives the distance of the number from the origin, $\left(0,\text{ }0\right)$.

### Example 2: Finding the Absolute Value of a Complex Number with a Radical

Find the absolute value of $z=\sqrt{5}-i$.

### Try It

Find the absolute value of the complex number $z=12 - 5i$.

### Example 3: Finding the Absolute Value of a Complex Number

Given $z=3 - 4i$, find $|z|$.

### Try It

Given $z=1 - 7i$, find $|z|$.

## Writing Complex Numbers in Polar Form

The polar form of a complex number expresses a number in terms of an angle $\theta$ and its distance from the origin $r$. Given a complex number in rectangular form expressed as $z=x+yi$, we use the same conversion formulas as we do to write the number in trigonometric form:

$\begin{gathered}x=r\cos \theta \\ y=r\sin \theta \\ r=\sqrt{{x}^{2}+{y}^{2}} \end{gathered}$

We review these relationships in Figure 5.

Figure 5

We use the term modulus to represent the absolute value of a complex number, or the distance from the origin to the point $\left(x,y\right)$. The modulus, then, is the same as $r$, the radius in polar form. We use $\theta$ to indicate the angle of direction (just as with polar coordinates). Substituting, we have

\begin{align}&z=x+yi \\ &z=r\cos \theta +\left(r\sin \theta \right)i \\ &z=r\left(\cos \theta +i\sin \theta \right) \end{align}

### A General Note: Polar Form of a Complex Number

Writing a complex number in polar form involves the following conversion formulas:

$\begin{gathered} x=r\cos \theta \\ y=r\sin \theta \\ r=\sqrt{{x}^{2}+{y}^{2}} \end{gathered}$

Making a direct substitution, we have

\begin{align}&z=x+yi \\ &z=\left(r\cos \theta \right)+i\left(r\sin \theta \right) \\ &z=r\left(\cos \theta +i\sin \theta \right) \end{align}

where $r$ is the modulus and $\theta$ is the argument. We often use the abbreviation $r\text{cis}\theta$ to represent $r\left(\cos \theta +i\sin \theta \right)$.

### Example 4: Expressing a Complex Number Using Polar Coordinates

Express the complex number $4i$ using polar coordinates.

### Try It

Express $z=3i$ as $r\text{cis}\theta$ in polar form.

### Example 5: Finding the Polar Form of a Complex Number

Find the polar form of $-4+4i$.

### Try It

Write $z=\sqrt{3}+i$ in polar form.

## Converting a Complex Number from Polar to Rectangular Form

Converting a complex number from polar form to rectangular form is a matter of evaluating what is given and using the distributive property. In other words, given $z=r\left(\cos \theta +i\sin \theta \right)$, first evaluate the trigonometric functions $\cos \theta$ and $\sin \theta$. Then, multiply through by $r$.

### Example 6: Converting from Polar to Rectangular Form

Convert the polar form of the given complex number to rectangular form:

$z=12\left(\cos \left(\frac{\pi }{6}\right)+i\sin \left(\frac{\pi }{6}\right)\right)$

### Example 7: Finding the Rectangular Form of a Complex Number

Find the rectangular form of the complex number given $r=13$ and $\tan \theta =\frac{5}{12}$.

### Try It

Convert the complex number to rectangular form:

$z=4\left(\cos \frac{11\pi }{6}+i\sin \frac{11\pi }{6}\right)$

## Finding Products and Quotients of Complex Numbers in Polar Form

Now that we can convert complex numbers to polar form we will learn how to perform operations on complex numbers in polar form. For the rest of this section, we will work with formulas developed by French mathematician Abraham de Moivre (1667-1754). These formulas have made working with products, quotients, powers, and roots of complex numbers much simpler than they appear. The rules are based on multiplying the moduli and adding the arguments.

### A General Note: Products of Complex Numbers in Polar Form

If ${z}_{1}={r}_{1}\left(\cos {\theta }_{1}+i\sin {\theta }_{1}\right)$ and ${z}_{2}={r}_{2}\left(\cos {\theta }_{2}+i\sin {\theta }_{2}\right)$, then the product of these numbers is given as:

\begin{align}{z}_{1}{z}_{2}&={r}_{1}{r}_{2}\left[\cos \left({\theta }_{1}+{\theta }_{2}\right)+i\sin \left({\theta }_{1}+{\theta }_{2}\right)\right] \\ {z}_{1}{z}_{2}&={r}_{1}{r}_{2}\text{cis}\left({\theta }_{1}+{\theta }_{2}\right) \end{align}

Notice that the product calls for multiplying the moduli and adding the angles.

### Example 8: Finding the Product of Two Complex Numbers in Polar Form

Find the product of ${z}_{1}{z}_{2}$, given ${z}_{1}=4\left(\cos \left(80^\circ \right)+i\sin \left(80^\circ \right)\right)$ and ${z}_{2}=2\left(\cos \left(145^\circ \right)+i\sin \left(145^\circ \right)\right)$.

## Finding Quotients of Complex Numbers in Polar Form

The quotient of two complex numbers in polar form is the quotient of the two moduli and the difference of the two arguments.

### A General Note: Quotients of Complex Numbers in Polar Form

If ${z}_{1}={r}_{1}\left(\cos {\theta }_{1}+i\sin {\theta }_{1}\right)$ and ${z}_{2}={r}_{2}\left(\cos {\theta }_{2}+i\sin {\theta }_{2}\right)$, then the quotient of these numbers is

\begin{align}&\frac{{z}_{1}}{{z}_{2}}=\frac{{r}_{1}}{{r}_{2}}\left[\cos \left({\theta }_{1}-{\theta }_{2}\right)+i\sin \left({\theta }_{1}-{\theta }_{2}\right)\right],{z}_{2}\ne 0\\ &\frac{{z}_{1}}{{z}_{2}}=\frac{{r}_{1}}{{r}_{2}}\text{cis}\left({\theta }_{1}-{\theta }_{2}\right),{z}_{2}\ne 0\end{align}

Notice that the moduli are divided, and the angles are subtracted.

### How To: Given two complex numbers in polar form, find the quotient.

1. Divide $\frac{{r}_{1}}{{r}_{2}}$.
2. Find ${\theta }_{1}-{\theta }_{2}$.
3. Substitute the results into the formula: $z=r\left(\cos \theta +i\sin \theta \right)$. Replace $r$ with $\frac{{r}_{1}}{{r}_{2}}$, and replace $\theta$ with ${\theta }_{1}-{\theta }_{2}$.
4. Calculate the new trigonometric expressions and multiply through by $r$.

### Example 9: Finding the Quotient of Two Complex Numbers

Find the quotient of ${z}_{1}=2\left(\cos \left(213^\circ \right)+i\sin \left(213^\circ \right)\right)$ and ${z}_{2}=4\left(\cos \left(33^\circ \right)+i\sin \left(33^\circ \right)\right)$.

### Try It

Find the product and the quotient of ${z}_{1}=2\sqrt{3}\left(\cos \left(150^\circ \right)+i\sin \left(150^\circ \right)\right)$ and ${z}_{2}=2\left(\cos \left(30^\circ \right)+i\sin \left(30^\circ \right)\right)$.

## Finding Powers and Roots of Complex Numbers in Polar Form

Finding powers of complex numbers is greatly simplified using De Moivre’s Theorem. It states that, for a positive integer $n,{z}^{n}$ is found by raising the modulus to the $n\text{th}$ power and multiplying the argument by $n$. It is the standard method used in modern mathematics.

### A General Note: De Moivre’s Theorem

If $z=r\left(\cos \theta +i\sin \theta \right)$ is a complex number, then

\begin{align}&{z}^{n}={r}^{n}\left[\cos \left(n\theta \right)+i\sin \left(n\theta \right)\right]\\ &{z}^{n}={r}^{n}\text{cis}\left(n\theta \right)\end{align}

where $n$ is a positive integer.

### Example 10: Evaluating an Expression Using De Moivre’s Theorem

Evaluate the expression ${\left(1+i\right)}^{5}$ using De Moivre’s Theorem.

## Finding Roots of Complex Numbers in Polar Form

To find the nth root of a complex number in polar form, we use the $n\text{th}$ Root Theorem or De Moivre’s Theorem and raise the complex number to a power with a rational exponent. There are several ways to represent a formula for finding $n\text{th}$ roots of complex numbers in polar form.

### A General Note: The nth Root Theorem

To find the $n\text{th}$ root of a complex number in polar form, use the formula given as

\begin{align}{z}^{\frac{1}{n}}={r}^{\frac{1}{n}}\left[\cos \left(\frac{\theta }{n}+\frac{2k\pi }{n}\right)+i\sin \left(\frac{\theta }{n}+\frac{2k\pi }{n}\right)\right]\end{align}

where $k=0,1,2,3,…,n - 1$. We add $\frac{2k\pi }{n}$ to $\frac{\theta }{n}$ in order to obtain the periodic roots.

### Example 11: Finding the nth Root of a Complex Number

Evaluate the cube roots of $z=8\left(\cos \left(\frac{2\pi }{3}\right)+i\sin \left(\frac{2\pi }{3}\right)\right)$.

### Try It

Find the four fourth roots of $16\left(\cos \left(120^\circ \right)+i\sin \left(120^\circ \right)\right)$.

## Key Concepts

• Complex numbers in the form $a+bi$ are plotted in the complex plane similar to the way rectangular coordinates are plotted in the rectangular plane. Label the x-axis as the real axis and the y-axis as the imaginary axis.
• The absolute value of a complex number is the same as its magnitude. It is the distance from the origin to the point: $|z|=\sqrt{{a}^{2}+{b}^{2}}$.
• To write complex numbers in polar form, we use the formulas $x=r\cos \theta ,y=r\sin \theta$, and $r=\sqrt{{x}^{2}+{y}^{2}}$. Then, $z=r\left(\cos \theta +i\sin \theta \right)$.
• To convert from polar form to rectangular form, first evaluate the trigonometric functions. Then, multiply through by $r$.
• To find the product of two complex numbers, multiply the two moduli and add the two angles. Evaluate the trigonometric functions, and multiply using the distributive property.
• To find the quotient of two complex numbers in polar form, find the quotient of the two moduli and the difference of the two angles.
• To find the power of a complex number ${z}^{n}$, raise $r$ to the power $n$, and multiply $\theta$ by $n$.
• Finding the roots of a complex number is the same as raising a complex number to a power, but using a rational exponent.

## Glossary

argument
the angle associated with a complex number; the angle between the line from the origin to the point and the positive real axis
De Moivre’s Theorem
formula used to find the $n\text{th}$ power or nth roots of a complex number; states that, for a positive integer $n,{z}^{n}$ is found by raising the modulus to the $n\text{th}$ power and multiplying the angles by $n$
modulus
the absolute value of a complex number, or the distance from the origin to the point $\left(x,y\right)$; also called the amplitude
polar form of a complex number
a complex number expressed in terms of an angle $\theta$ and its distance from the origin $r$; can be found by using conversion formulas $x=r\cos \theta ,y=r\sin \theta$, and $r=\sqrt{{x}^{2}+{y}^{2}}$

## Section 6.5 Homework Exercises

1. What is a system of parametric equations?

2. Some examples of a third parameter are time, length, speed, and scale. Explain when time is used as a parameter.

3. Explain how to eliminate a parameter given a set of parametric equations.

4. What is a benefit of writing a system of parametric equations as a Cartesian equation?

5. What is a benefit of using parametric equations?

6. Why are there many sets of parametric equations to represent on Cartesian function?

For the following exercises, eliminate the parameter $t$ to rewrite the parametric equation as a Cartesian equation.

7. $\begin{cases}x\left(t\right)=5-t\hfill \\ y\left(t\right)=8 - 2t\hfill \end{cases}$

8. $\begin{cases}x\left(t\right)=6 - 3t\hfill \\ y\left(t\right)=10-t\hfill \end{cases}$

9. $\begin{cases}x\left(t\right)=2t+1\hfill \\ y\left(t\right)=3\sqrt{t}\hfill \end{cases}$

10. $\begin{cases}x\left(t\right)=3t - 1\hfill \\ y\left(t\right)=2{t}^{2}\hfill \end{cases}$

11. $\begin{cases}x\left(t\right)=2{e}^{t}\hfill \\ y\left(t\right)=1 - 5t\hfill \end{cases}$

12. $\begin{cases}x\left(t\right)={e}^{-2t}\hfill \\ y\left(t\right)=2{e}^{-t}\hfill \end{cases}$

13. $\begin{cases}x\left(t\right)=4\text{log}\left(t\right)\hfill \\ y\left(t\right)=3+2t\hfill \end{cases}$

14. $\begin{cases}x\left(t\right)=\text{log}\left(2t\right)\hfill \\ y\left(t\right)=\sqrt{t - 1}\hfill \end{cases}$

15. $\begin{cases}x\left(t\right)={t}^{3}-t\hfill \\ y\left(t\right)=2t\hfill \end{cases}$

16. $\begin{cases}x\left(t\right)=t-{t}^{4}\hfill \\ y\left(t\right)=t+2\hfill \end{cases}$

17. $\begin{cases}x\left(t\right)={e}^{2t}\hfill \\ y\left(t\right)={e}^{6t}\hfill \end{cases}$

18. $\begin{cases}x\left(t\right)={t}^{5}\hfill \\ y\left(t\right)={t}^{10}\hfill \end{cases}$

19. $\begin{cases}x\left(t\right)=4\text{cos}t\hfill \\ y\left(t\right)=5\sin t \hfill \end{cases}$

20. $\begin{cases}x\left(t\right)=3\sin t\hfill \\ y\left(t\right)=6\cos t\hfill \end{cases}$

21. $\begin{cases}x\left(t\right)=2{\text{cos}}^{2}t\hfill \\ y\left(t\right)=-\sin t \hfill \end{cases}$

22. $\begin{cases}x\left(t\right)=\cos t+4\\ y\left(t\right)=2{\sin }^{2}t\end{cases}$

23. $\begin{cases}x\left(t\right)=t - 1\\ y\left(t\right)={t}^{2}\end{cases}$

24. $\begin{cases}x\left(t\right)=-t\\ y\left(t\right)={t}^{3}+1\end{cases}$

25. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)={t}^{3}-2\end{cases}$

For the following exercises, rewrite the parametric equation as a Cartesian equation by building an $x\text{-}y$ table.

26. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)=t+4\end{cases}$

27. $\begin{cases}x\left(t\right)=4-t\\ y\left(t\right)=3t+2\end{cases}$

28. $\begin{cases}x\left(t\right)=2t - 1\\ y\left(t\right)=5t\end{cases}$

29. $\begin{cases}x\left(t\right)=4t - 1\\ y\left(t\right)=4t+2\end{cases}$

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by setting $x\left(t\right)=t$ or by setting $y\left(t\right)=t$.

30. $y\left(x\right)=3{x}^{2}+3$

31. $y\left(x\right)=2\sin x+1$

32. $x\left(y\right)=3\mathrm{log}\left(y\right)+y$

33. $x\left(y\right)=\sqrt{y}+2y$

For the following exercises, parameterize (write parametric equations for) each Cartesian equation by using $x\left(t\right)=a\cos t$ and $y\left(t\right)=b\sin t$. Identify the curve.

34. $\frac{{x}^{2}}{4}+\frac{{y}^{2}}{9}=1$

35. $\frac{{x}^{2}}{16}+\frac{{y}^{2}}{36}=1$

36. ${x}^{2}+{y}^{2}=16$

37. ${x}^{2}+{y}^{2}=10$

38. Parameterize the line from $\left(3,0\right)$ to $\left(-2,-5\right)$ so that the line is at $\left(3,0\right)$ at $t=0$, and at $\left(-2,-5\right)$ at $t=1$.

39. Parameterize the line from $\left(-1,0\right)$ to $\left(3,-2\right)$ so that the line is at $\left(-1,0\right)$ at $t=0$, and at $\left(3,-2\right)$ at $t=1$.

40. Parameterize the line from $\left(-1,5\right)$ to $\left(2,3\right)$ so that the line is at $\left(-1,5\right)$ at $t=0$, and at $\left(2,3\right)$ at $t=1$.

41. Parameterize the line from $\left(4,1\right)$ to $\left(6,-2\right)$ so that the line is at $\left(4,1\right)$ at $t=0$, and at $\left(6,-2\right)$ at $t=1$.

For the following exercises, use the table feature in the graphing calculator to determine whether the graphs intersect.

42. $\begin{cases}{x}_{1}\left(t\right)=3t\hfill \\ {y}_{1}\left(t\right)=2t - 1\hfill \end{cases}\text{ and }\begin{cases}{x}_{2}\left(t\right)=t+3\hfill \\ {y}_{2}\left(t\right)=4t - 4\hfill \end{cases}$

43. $\begin{cases}{x}_{1}\left(t\right)={t}^{2}\hfill \\ {y}_{1}\left(t\right)=2t - 1\hfill \end{cases}\text{ and }\begin{cases}{x}_{2}\left(t\right)=-t+6\hfill \\ {y}_{2}\left(t\right)=t+1\hfill \end{cases}$

For the following exercises, use a graphing calculator to complete the table of values for each set of parametric equations.

44. $\begin{cases}{x}_{1}\left(t\right)=3{t}^{2}-3t+7\hfill \\ {y}_{1}\left(t\right)=2t+3\hfill \end{cases}$

$t$ $x$ $y$
–1
0
1

45. $\begin{cases}{x}_{1}\left(t\right)={t}^{2}-4\hfill \\ {y}_{1}\left(t\right)=2{t}^{2}-1\hfill \end{cases}$

$t$ $x$ $y$
1
2
3

46. $\begin{cases}{x}_{1}\left(t\right)={t}^{4}\hfill \\ {y}_{1}\left(t\right)={t}^{3}+4\hfill \end{cases}$

$t$ $x$ $y$
-1
0
1
2

47. Find two different sets of parametric equations for $y={\left(x+1\right)}^{2}$.

48. Find two different sets of parametric equations for $y=3x - 2$.

49. Find two different sets of parametric equations for $y={x}^{2}-4x+4$.