# Chapter 6: The Polar System

**At Grade**Created by: CK-12

- 6.1.
## Polar Coordinates

- 6.2.
## Graphing Basic Polar Equations

- 6.3.
## Converting Between Systems

- 6.4.
## More with Polar Curves

- 6.5.
## The Trigonometric Form of Complex Numbers

- 6.6.
## The Product & Quotient Theorems

- 6.7.
## De Moivre’s and the nth Root Theorems

### Chapter Summary

In this chapter we made the connection between complex numbers and trigonometry. First, we started with the polar system, by graphing and converting equations into polar coordinates. This allowed us to compare the complex plane with the polar plane and we realized that there are many similarities. Because of this, we are able to convert complex numbers into polar, or trigonometric, form. Converting complex numbers to polar form makes it easier to multiply and divide complex numbers by using the Product and Quotient theorems. These theorems lead to De Moivre’s Theorem, which is a shortcut for raising complex numbers to different powers. Finally, we were able manipulate De Moivre’s Theorem to find all the complex solutions to different equations.

## Vocabulary

- Argument
- In the complex number
r(cosθ+isinθ) , the argument is the angleθ .

- Modulus
- In the complex number
r(cosθ+isinθ) , the modulus isr . It is the distance from the origin to the point(x,y) in the complex plane.

- Polar coordinate system
- A method of recording the position of an object by using the distance from a fixed point and an angle consisting of a fixed ray from that point. Also called a polar plane.

- Pole
- In a polar coordinate system, it is the fixed point or origin.

- Polar axis
- In a polar coordinate system, it is the horizontal ray that begins at the pole and extends in a positive direction.

- Polar coordinates
- The coordinates of a point plotted on a polar plane
(r,θ) .

- Polar Equation
- An equation which uses polar coordinates.

- Polar Form
*Also*called trigonometric form is the complex numberx+yi written asr(cosθ+isinθ) wherer=x2+y2−−−−−−√ andtanθ=yx .

## Review Questions

- Plot
A(−3,3π4) and find three other equivalent coordinates. - Find the distance between
(2,94∘) and(7,−73∘) . - Graph the following polar curves.
r=3sin5θ r=6−3cosθ r=2+5cos9θ

- Determine the equations of the curves below.
- Convert each equation or point into polar form.
A(−6,11) B(15,−8) C(9,40) x2+(y−6)2=36

- Convert each equation or point into rectangular form.
D(4,−π3) E(−2,135∘) r=7 r=8sinθ

- Determine where
r=6+5sinθ andr=3−4cosθ intersect. - Change
−3+8i into polar form. - Change
15∠240∘ into rectangular form. - Multiply or divide the following complex numbers using polar form.
(7cis7π4)⋅(3cisπ3) 8∠80∘2∠−155∘

- Expand
[4(cosπ4+isinπ4)]6 - Find the
6th roots of -64 and graph them in the complex plane. - Find all the solutions of
x4+32=0 .

## Texas Instruments Resources

*In the CK-12 Texas Instruments Trigonometry FlexBook® resource, there are graphing calculator activities designed to supplement the objectives for some of the lessons in this chapter. See http://www.ck12.org/flexr/chapter/9704.*

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