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Graphs of Polar Equations
In the last section, we learned how to graph a point with polar coordinates (r, θ). We will now look at graphing polar equations. Just as a quick review, the polar coordinate system is very similar to that of the rectangular coordinate system. In a polar coordinate grid, as shown below, there will be a series of circles extending out from the pole (or origin in a rectangular coordinate grid) and five different lines passing through the pole to represent the angles at which the exact values are known for the trigonometric functions.
Graphing a polar equation is accomplished in pretty much the same manner as rectangular equations are graphed. They can be graphed by point-plotting, using the trigonometric functions period, and using the equation’s symmetry (if any). When graphing rectangular equations by point-plotting you would pick values for x and then evaluate the equation to determine its corresponding y value. For a polar equation, you would pick angle measurements for θ and then evaluate the equation to determine its corresponding r value.
Symmetry tests for polar coordinates
- Replace θ with -θ. If an equivalent equation results, the graph is symmetric with respect to the polar axis.
- Replace θ with -θ and r with -r. If an equivalent equation results, the graph is symmetric with respect to θ = 2
- Replace r with -r. If an equivalent equation results, the graph is symmetric with respect to the pole.
Note: It is possible for a polar equation to fail a test and still exhibit that type of symmetry when you finish graphing the function over a full period.
When you started to graph functions (in rectangular form) you stared by learning the basic shapes of certain functions such as lines, parabolas, circles, square roots, and absolute value functions just to name a few. Polar equations also have some general types of equations. Learning to recognize the formulas of these equations will help in sketching the graphs.
Circles in Polar Form
- r = a cos θ is a circle where “a” is the diameter of the circle that has its left-most edge at the pole.
- r = a sin θ is a circle where “a” is the diameter of the circle that has its bottom-most edge at the pole.
Rose Curves
A rose curve is a graph that is produced from a polar equation in the form of:
r = a sin nθ or r = a cos nθ, where a ≠ 0 and n is an integer > 1
They are called rose curves because the loops that are formed resemble petals. The number of petals that are present will depend on the value of n. The value of a will determine the length of the petals.
If n is an even integer, then the rose will have 2n petals.
r = a sin 2θ r = a cos 4θ
If n is an odd integer, then the rose will have n petals.
r = a sin 5θ r = a cos 3θ
Lemniscates
The last type of polar equation that we will cover here is the lemniscates, which has the shape of a figure-8 or a propeller. Lemniscates have the general polar equation of:
r 2 = a^2 sin 2θ or r 2 = a^2 cos 2θ, where a ≠ 0
A lemniscate containing the sine function will be symmetric to the pole while the lemniscate
containing the cosine function will be symmetric to the polar axis, to θ = 2
π , and the pole.
r 2 = a^2 sin 2θ r 2 = a^2 cos 2θ
Example 1: Graph the polar equation r = 1 – 2 cos θ.
Solution:
Identify the type of polar equation
The polar equation is in the form of a limaçon, r = a – b cos θ.
Find the ratio of a b
to determine the equation’s general shape
a b
Since the ratio is less than 1, it will have both an inner and outer loop. The loops will be along the polar axis since the function is cosine and will loop to the left since the sign between a and b is minus.
Example 1 (Continued):
θ r = 1 – 2 cos θ (r, θ) 2 3
π 1 – 2 cos 2
π 1 – 2 cos 3
π 1 – 2 cos 5
π 1 – 2 cos π = 1 – 2 * -1 = 1 + 2 = 3
(π, 3)
Plot the points
Example 1 (Continued):
Use the symmetry to complete the graph
Example 2: Graph the polar equation r = 3 cos 2θ.
Solution:
Identify the type of polar equation
The polar equation is in the form of a rose curve, r = a cos nθ. Since n is an even integer, the rose will have 2n petals.
2n = 2(2) = 4 petals
Test for symmetry
Polar axis θ = 2
π Pole
r = 3 cos 2θ r = 3 cos 2(-θ) r = 3 cos (-2θ) r = 3 cos 2θ
r = 3 cos 2θ -r = 3 cos 2(-θ) r = -3 cos (-2θ) r = –3 cos 2θ
r = 3 cos 2θ -r = 3 cos 2θ r = –3 cos 2θ
Passes symmetry test Fails symmetry test Fails symmetry test
Example 2 (Continued):
5 6
π 3 cos 2( 5
3 cos 5 3
π 3 cos 2(π) = 3 * 1 = 3
(π, 3)
These points will provide us with enough points to complete the rest of the graph using the symmetry of the rose curve.
Plot the points
Example 2 (Continued):
Use the symmetry to complete the graph
