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Math 112Elementary Functions
Section 4
Polar Coordinates and Graphs
Chapter 7 – Applications of Trigonometry
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Rectangular (aka: Cartesian) Coordinates
positive x-axisnegative x-axis
positive y-axis
negative y-axis
x
y(x, y)
origin
For any point there is a unique ordered pair (x, y) that
specifies the location of that point.
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Polar Coordinates
polar axis
(r, )
r
pole
Is (r, ) unique for
every point?
NO!
All of the following refer to the same point:
(5, 120º)(5, 480º)(-5, 300º)(-5, -60º)etc ...
The angle may be expressed in degrees or radians.
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Polar Graph PaperLocating and Graphing Points
0
30
6090
180
120
150
210
240270
300
330
(5, 150) (6, 75)
(3, 300)(3, -60)(-3, 120)
(-4, 30)
(7, 0)
(-7, 180)
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Converting CoordinatesPolar Rectangular
222 yxr
x
ytan
sinry
x
y(r, ) (x, y)
r
Recommendation: Find (r, ) wherer > 0 and0 ≤ < 2 or 0 ≤ < 360.
cosrx
Relationships between r, , x, & y
R P
P R
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Examples: Converting CoordinatesPolar Rectangular
sinry cosrx
)210 ,3(
)210sin3 ,210cos3(
2
13 ,
2
33
2
3 ,
2
33
6 ,2
6sin2 ,
6cos2
2
12 ,
2
32
1- ,3
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Examples: Converting CoordinatesPolar Rectangular
222 yxr x
ytan
Quadrant I
)7 ,3( 5873 22 r
8.663
7tan 1
)8.66 ,58( )7 ,3(
3
7tan
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Examples: Converting CoordinatesPolar Rectangular
222 yxr x
ytan
Quadrant II
)7 ,3( 587)3( 22 r
8.663
7tan 1
)2.113 ,58()1808.66 ,58( )7 ,3(
3
7tan
)8.66 ,58( )7 ,3( OR
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Examples: Converting CoordinatesPolar Rectangular
222 yxr x
ytan
Quadrant III
)7 ,3( 58)7()3( 22 r
8.663
7tan 1
)8.246 ,58()1808.66 ,58( )7 ,3(
3
7tan
)8.66 ,58( )7 ,3( OR
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Examples: Converting CoordinatesPolar Rectangular
222 yxr x
ytan
Quadrant IV
)7 ,3( 58)7(3 22 r
8.663
7tan 1
)93.22 ,58()3608.66 ,58( )7 ,3(
3
7tan
)8.66 ,58( )7 ,3( OR
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Converting EquationsPolar Rectangular
Use the same identities:
222 yxr
x
ytan sinry
cosrx
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Converting EquationsPolar Rectangular
Replace all occurrences of xx with r cos .
Replace all occurrences of yy with r sin .
Simplify Solve for rr (if possible).
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Converting EquationsPolar Rectangular
Express the equation in terms of sine and cosine only.
If possible, manipulate the equation so that all occurrences of cos and sin are multiplied by r.
Replace all occurrences of …
Simplify (solve for y if possible)
r cos with x
r sin with y
r2 with x2 + y2
Or, if all else fails, use:
22cos
yx
x
22sin
yx
y
22 yxr
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Graphing Polar Equations
Reminder: How do you graph rectangular equations? Method 1:
Create a table of values. Plot ordered pairs. Connect the dots in order as x increases.
Method 2: Recognize and graph various common forms. Examples: linear equations, quadratic equations, conics, …
The same basic approach can be applied to polar equations.
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Graphing Polar EquationsMethod 1: Plotting and Connecting Points
1. Create a table of values.
2. Plot ordered pairs.
3. Connect the dots in order as increases.
NOTE: Since most of these equations involve periodic functions (esp. sine and cosine), at some point the graph will start repeating itself (but not always).
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Circles Centered at the origin: r = a
radius: a period = 360
Tangent to the x-axis at the origin: r = a sin center: (a/2, 90) radius: a/2 period = 180 a > 0 above a < 0 below
Tangent to the y-axis at the origin: r = a cos center: (a/2, 90) radius: a/2 period = 180 a > 0 right a < 0 left
r = 4
r = 4 sin
r = 4 cos
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Flowers (centered at the origin) r = a cos n or r = a sin n
radius: |a| n is even 2n petals
petal every 180/n period = 360
n is odd n petals petal every 360/n period = 180
cos 1st petal @ 0 sin 1st petal @ 90/n
r = 4 sin 2
r = 4 cos 3
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Spirals Spiral of Archimedes: r = k
|k| large loose |k| small tight
r = r = ¼
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Heart (actually: cardioid if a = b … otherwise: limaçon)
r = a ± b cos or r = a ± b sin
r = 3 + 3 cos r = 2 - 5 cos r = 3 + 2 sin r = 3 - 3 sin
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Lines
Horizontal: y = k r sin = k r = k csc
Vertical: x = h r cos = h r = h sec
Others:
ax + by = c
y = mx + b
sincos ba
cr
cossin m
br
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Graphing Polar EquationsMethod 2: Recognizing Common Forms
Parabolas (w/ vertex on an axis)
NOTE: With these forms, the vertex will never be at the origin.
cos1
ar
sin1
ar
cos1
3
r
cos1
7
r
sin1
5
r
sin1
1
r