1. HW # −14 , ,18 , ,44 , Row 6
Do Now Convert the polar equation to rectangular form 𝑟=6 sin 𝜃

2. Do Now Solutions
Convert the polar equation to rectangular form 𝑟=6 sin 𝜃 𝑟=6 𝑦 𝑟 𝑟 2 =6𝑦 𝑥 2 + 𝑦 2 =6𝑦 𝑥 2 + 𝑦 2 −6𝑦=0 𝑥 2 + 𝑦 2 −6𝑦+9=9 𝑥 2 + 𝑦−3 2 =9

3. Graphs of Polar Equations

4. Objectives Graph polar equations by point plotting.
Use symmetry, zeros, and maximum r-values to sketch graphs of polar equations.
Recognize special polar graphs.

5. Example 1 – Graphing a Polar Equation by Point Plotting
Sketch the graph of the polar equation r = 4 sin . Solution: The sine function is periodic, so you can get a full range of r-values by considering values of  in the interval 0    2, as shown in the following table.

6. Example 1 – Solution
cont’d
By plotting these points, as shown in Figure 10.54, it appears that the graph is a circle of radius 2 whose center is at the point (x, y) = (0, 2).
Figure 10.54

7. You can confirm the graph in Figure 10
You can confirm the graph in Figure by converting the polar equation to rectangular form and then sketching the graph of the rectangular equation. You can also use a graphing calculator set to polar mode and graph the polar equation.

8. Symmetry, Zeros, and Maximum r-Values

9. Symmetry, Zeros, and Maximum r-Values
In Figure 10.54, note that as  increases from 0 to 2 the graph is traced out twice. Moreover, note that the graph is symmetric with respect to the line  =  /2.
Figure 10.54

10. Symmetry, Zeros, and Maximum r-Values
The three important types of symmetry to consider in polar curve sketching are shown below.
Symmetry with Respect to the Line  =
Symmetry with Respect to the Polar Axis
Symmetry with Respect to the Pole

11. Symmetry, Zeros, and Maximum r-Values

12. Example 2 – Using Symmetry to Sketch a Polar Graph
Use symmetry to sketch the graph of r = cos . Solution: Replacing (r,  ) by (r, – ) produces r = cos(– ) = cos . So, you can conclude that the curve is symmetric with respect to the polar axis.
cos(– ) = cos 

13. Example 2 – Solution
cont’d
Plotting the points in the table and using polar axis symmetry, you obtain the graph shown in Figure This graph is called a limaçon.
Figure 10.55

14. Symmetry, Zeros, and Maximum r-Values
The three tests for symmetry in polar coordinates are sufficient to guarantee symmetry, but they are not necessary. For instance, Figure shows the graph of r =  + 2 to be symmetric with respect to the line  =  /2, and yet the tests fail to indicate symmetry because neither of the following replacements yields an equivalent equation.
Spiral of Archimedes
Figure 10.56

15. Symmetry, Zeros, and Maximum r-Values
Original Equation Replacement New Equation r =  + 2 (r,  ) by (–r, – ) –r = – + 2 r =  + 2 (r,  ) by (r,  –  ) r = – + 3 The equations discussed in Examples 1 and 2 are of the form r = 4sin  = f (sin  ) and r = 3 + 2cos  = g(cos  ). The graph of the first equation is symmetric with respect to the line  =  /2, and the graph of the second equation is symmetric with respect to the polar axis.

16. Symmetry, Zeros, and Maximum r-Values
This observation can be generalized to yield the following tests.

17. Symmetry, Zeros, and Maximum r-Values
Two additional aids to sketching graphs of polar equations involve knowing the  -values for which | r | is maximum and knowing the  -values for which r = 0. For instance, in Example 1, the maximum value of | r | for r = 4 sin  is | r | = 4, and this occurs when  =  /2, as shown in Figure Moreover, when r = 0 when  = 0.
Figure 10.54

18. Example 3 – Sketching a Polar Graph
Sketch the graph of r = 1 – 2 cos . Solution: From the equation r = 1 – 2 cos , you can obtain the following. Symmetry: With respect to the polar axis Maximum value of | r |: r = 3 when  =  Zero of r : r = 0 when  =  /3

19. Example 3 – Solution
cont’d
The table shows several  -values in the interval [0,  ]. By plotting the corresponding points, you can sketch the graph shown in Figure
Figure 10.57

20. Example 3 – Solution
cont’d
Note how the negative r-values determine the inner loop of the graph in Figure This graph, like the one in Figure 10.55, is a limaçon.
Figure 10.55

21. Special Polar Graphs

22. Special Polar Graphs
Several important types of graphs have equations that are simpler in polar form than in rectangular form. For example, the circle r = 4 sin  in Example 1 has the more complicated rectangular equation x2 + (y – 2)2 = 4.

23. Special Polar Graphs
Several other types of graphs that have simple polar equations are shown below. Limaçons r = a  b cos , r = a  b sin , (a > 0, b > 0)

24. Special Polar Graphs
Rose Curves n petals when n is odd, 2n petals if n is even (n  2)

25. Special Polar Graphs
Circles and Lemniscates

26. Example 5 – Sketching a Rose Curve
Sketch the graph of r = 3 cos 2. Solution: Type of curve: Rose curve with 2n = 4 petals Symmetry: With respect to polar axis, the line  = , and the pole Maximum value of | r |: | r | = 3 when  = 0, ,  , Zero of r : r = 0 when  = ,

27. Example 5 – Solution
cont’d
Using this information together with the additional points shown in the following table, you obtain the graph shown in Figure
Figure 10.58