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How are the polar axes different from the x - and y -axes of the Cartesian plane?

Explain how polar coordinates are graphed.

Determine θ for the point, then move r units from the pole to plot the point. If r is negative, move r units from the pole in the opposite direction but along the same angle. The point is a distance of r away from the origin at an angle of θ from the polar axis.

How are the points ( 3 , π 2 ) and ( 3 , π 2 ) related?

Explain why the points ( 3 , π 2 ) and ( 3 , π 2 ) are the same.

The point ( 3 , π 2 ) has a positive angle but a negative radius and is plotted by moving to an angle of π 2 and then moving 3 units in the negative direction. This places the point 3 units down the negative y -axis. The point ( 3 , π 2 ) has a negative angle and a positive radius and is plotted by first moving to an angle of π 2 and then moving 3 units down, which is the positive direction for a negative angle. The point is also 3 units down the negative y -axis.

Algebraic

For the following exercises, convert the given polar coordinates to Cartesian coordinates with r > 0 and 0 θ 2 π . Remember to consider the quadrant in which the given point is located when determining θ for the point.

( 7 , 7 π 6 )

( 5 , π )

( 5 , 0 )

( 6 , π 4 )

( 3 , π 6 )

( 3 3 2 , 3 2 )

( 4 , 7 π 4 )

For the following exercises, convert the given Cartesian coordinates to polar coordinates with r > 0 , 0 θ < 2 π . Remember to consider the quadrant in which the given point is located.

( 4 , 2 )

( 2 5 ,   0.464 )

( 4 , 6 )

( 3 , −5 )

( 34 , 5.253 )

( −10 , −13 )

( 8 , 8 )

( 8 2 , π 4 )

For the following exercises, convert the given Cartesian equation to a polar equation.

x = 3

y = 4

r = 4 csc θ

y = 4 x 2

y = 2 x 4

r = s i n θ 2 c o s 4 θ 3

x 2 + y 2 = 4 y

x 2 + y 2 = 3 x

r = 3 cos θ

x 2 y 2 = x

x 2 y 2 = 3 y

r = 3 sin θ cos ( 2 θ )

x 2 + y 2 = 9

x 2 = 9 y

r = 9 sin θ cos 2 θ

y 2 = 9 x

9 x y = 1

r = 1 9 cos θ sin θ

For the following exercises, convert the given polar equation to a Cartesian equation. Write in the standard form of a conic if possible, and identify the conic section represented.

r = 3 sin θ

r = 4 cos θ

x 2 + y 2 = 4 x or ( x 2 ) 2 4 + y 2 4 = 1 ; circle

r = 4 sin θ + 7 cos θ

r = 6 cos θ + 3 sin θ

3 y + x = 6 ; line

r = 2 sec θ

r = 3 csc θ

y = 3 ; line

r = r cos θ + 2

r 2 = 4 sec θ csc θ

x y = 4 ; hyperbola

r = 4

r 2 = 4

x 2 + y 2 = 4 ; circle

r = 1 4 cos θ 3 sin θ

r = 3 cos θ 5 sin θ

x 5 y = 3 ; line

Graphical

For the following exercises, find the polar coordinates of the point.

Polar coordinate system with a point located on the third concentric circle and pi/2.
Polar coordinate system with a point located on the third concentric circle and midway between pi/2 and pi in the second quadrant.

( 3 , 3 π 4 )

Polar coordinate system with a point located midway between the first and second concentric circles and a third of the way between pi and 3pi/2 (closer to pi).
Polar coordinate system with a point located on the fifth concentric circle and pi.

( 5 , π )

Polar coordinate system with a point located on the fourth concentric circle and a third of the way between 3pi/2 and 2pi (closer to 3pi/2).

For the following exercises, plot the points.

( 2 , π 3 )

Polar coordinate system with a point located on the second concentric circle and two-thirds of the way between pi and 3pi/2 (closer to 3pi/2).

( 1 , π 2 )

( 3.5 , 7 π 4 )

Polar coordinate system with a point located midway between the third and fourth concentric circles and midway between 3pi/2 and 2pi.

( 4 , π 3 )

( 5 , π 2 )

Polar coordinate system with a point located on the fifth concentric circle and pi/2.

( 4 , 5 π 4 )

( 3 , 5 π 6 )

Polar coordinate system with a point located on the third concentric circle and 2/3 of the way between pi/2 and pi (closer to pi).

( 1.5 , 7 π 6 )

( 2 , π 4 )

Polar coordinate system with a point located on the second concentric circle and midway between pi and 3pi/2.

( 1 , 3 π 2 )

For the following exercises, convert the equation from rectangular to polar form and graph on the polar axis.

5 x y = 6

r = 6 5 cos θ sin θ

Plot of given line in the polar coordinate grid

2 x + 7 y = 3

x 2 + ( y 1 ) 2 = 1

r = 2 sin θ

Plot of given circle in the polar coordinate grid

( x + 2 ) 2 + ( y + 3 ) 2 = 13

x = 2

r = 2 cos θ

Plot of given circle in the polar coordinate grid

x 2 + y 2 = 5 y

x 2 + y 2 = 3 x

r = 3 cos θ

Plot of given circle in the polar coordinate grid.

For the following exercises, convert the equation from polar to rectangular form and graph on the rectangular plane.

r = 6

r = 4

x 2 + y 2 = 16

Plot of circle with radius 4 centered at the origin in the rectangular coordinates grid.

θ = 2 π 3

θ = π 4

y = x

Plot of line y=x in the rectangular coordinates grid.

r = sec θ

r = −10 sin θ

x 2 + ( y + 5 ) 2 = 25

Plot of circle with radius 5 centered at (0,-5).

r = 3 cos θ

Technology

Use a graphing calculator to find the rectangular coordinates of ( 2 , π 5 ) . Round to the nearest thousandth.

( 1.618 , 1.176 )

Use a graphing calculator to find the rectangular coordinates of ( 3 , 3 π 7 ) . Round to the nearest thousandth.

Use a graphing calculator to find the polar coordinates of ( 7 , 8 ) in degrees. Round to the nearest thousandth.

( 10.630 , 131.186° )

Use a graphing calculator to find the polar coordinates of ( 3 , 4 ) in degrees. Round to the nearest hundredth.

Use a graphing calculator to find the polar coordinates of ( 2 , 0 ) in radians. Round to the nearest hundredth.

( 2 , 3.14 ) o r ( 2 , π )

Extensions

Describe the graph of r = a sec θ ; a > 0.

Describe the graph of r = a sec θ ; a < 0.

A vertical line with a units left of the y -axis. 

Describe the graph of r = a csc θ ; a > 0.

Describe the graph of r = a csc θ ; a < 0.

A horizontal line with a units below the x -axis.

What polar equations will give an oblique line?

For the following exercise, graph the polar inequality.

r < 4

Graph of shaded circle of radius 4 with the edge not included (dotted line) - polar coordinate grid.

0 θ π 4

θ = π 4 , r 2

Graph of ray starting at (2, pi/4) and extending in a positive direction along pi/4 - polar coordinate grid.

θ = π 4 , r −3

0 θ π 3 , r < 2

Graph of the shaded region 0 to pi/3 from r=0 to 2 with the edge not included (dotted line) - polar coordinate grid

π 6 < θ π 3 , 3 < r < 2

Practice Key Terms 3

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Source:  OpenStax, Essential precalculus, part 2. OpenStax CNX. Aug 20, 2015 Download for free at http://legacy.cnx.org/content/col11845/1.2
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