# Polar Equations

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A polar rose (Rhodonea Curve)
This polar rose is created with the polar equation: $r = cos(\pi\theta)$.
A polar rose (Rhodonea Curve)
Fields: Algebra and Calculus
Created By: chanj

# Basic Description

Polar equations are used to create interesting curves, and in most cases they are periodic like sine waves. Other types of curves can also be created using polar equations besides roses, such as Archimedean spirals and limaçons. See the Polar Coordinates page for some background information.

# A More Mathematical Explanation

Note: understanding of this explanation requires: *calculus, trigonometry
[Click to view A More Mathematical Explanation]

## Rose

The general polar equations form to create a rose is UNIQ5c9d636c480d0bf4-math-00000001-Q [...]

[Click to hide A More Mathematical Explanation]

## Rose

The general polar equations form to create a rose is $r = a \sin(n \theta)$ or $r = a \cos(n \theta)$. Note that the difference between sine and cosine is $\sin(\theta) = \cos(\theta-\frac{\pi}{2})$, so choosing between sine and cosine affects where the curve starts and ends. $a$ represents the maxium value $r$ can be, i.e. the maximum radius of the rose. $n$ affects the number of petals on the graph:

• If $n$ is an odd integer, then there would be $n$ petals, and the curve repeats itself every $\pi$.
• If $n$ is an even integer, then there would be $2n$ petals, and the curve repeats itself every $2 \pi$.
• If $n$ is a rational fraction ($p/q$ where $p$ and $q$ are integers), then the curve repeats at the $\theta = \pi q k$, where $k = 1$ if $pq$ is odd, and $k = 2$ if $pq$ is even.
• If $n$ is irrational, then there are an infinite number of petals.

Below is an applet to graph polar roses:

If you can see this message, you do not have the Java software required to view the applet.

## Other Polar Curves

Archimedean Spirals

Archimedes' Spiral $r = a\theta$  The spiral can be used to square a circle and trisect an angle.                                       Fermat's Spiral $r = \pm a\sqrt\theta$   This spiral's pattern can be seen in disc phyllotaxis.
Hyperbolic spiral$r = \frac{a}{\theta}$  It begins at an infinite distance from the pole, and winds faster as it approaches closer to the pole.  Lituus $r^2 \theta = a^2$It is asymptotic at the $x$ axis as the distance increases from the pole.

Limaçon[1]
The word "limaçon" derives from the Latin word "limax," meaning snail. The general equation for a limaçon is $r = b + a\cos(\theta)$.

• If $b \geq 2a$, then the curve is convex.
• If $2a > b > a$, then it is dimpled.
• If $b = a$, then it becomes a cardioid.
• If $b = a/2$, then it is a trisectrix.

$r = \cos(\theta)$  $r = 0.5 + \cos(\theta)$   |  Cardioid $r = 1 + \cos(\theta)$  $r = 2 + \cos(\theta)$

## Finding Derivatives

Consider the polar curve $r = f(\theta)$.
If we turn it into parametric equations, we would get:

• $x = r \cos(\theta) = f(\theta) \cos(\theta)$
• $y = r \sin(\theta) = f(\theta) \sin(\theta)$

Using the method of finding the derivative of parametric equations and the product rule, we would get:
$\frac{dy}{dx} = \frac{\frac{dr}{d\theta} \sin(\theta) + r \cos(\theta)}{\frac{dr}{d\theta} \cos(\theta) - r \sin(\theta)}$

## Finding Areas and Arc Lengths

To find the area of a sector of a circle, where $r$ is the radius, you would use $A = \frac{1}{2} r^2 \theta$.
Therefore, for $r = f(\theta)$, the formula for the area of a polar region is:

$A = \int_a^b\! \frac{1}{2} r^2 d\theta$

The formula to find the arc length for $r = f(\theta)$ and assuming $r$ is continuous is:

$L = \int_a^b\! \sqrt{r^2 + {\bigg(\frac{dr}{d\theta}\bigg)} ^2}$ $d\theta$

# Why It's Interesting

Polar coordinates are often used in navigation, such as aircrafts. They are also used to plot gravitational fields and point sources. Furthermore, polar patterns are seen in the directionality of microphones, which is the direction at which the microphone picks up sound. A well-known pattern is a cardioid.

## Possible Future work

• More details can be written about the different curves, maybe they can get their own pages.
• Applets can be made to draw these different curves, like the one on the page for roses.

# Teaching Materials

There are currently no teaching materials for this page. Add teaching materials.

# Related Links

### Additional Resources

Polar Coordinates
Cardioid

# References

Wolfram MathWorld: Rose, Limacon, Archimedean Spiral
Wikipedia: Polar Coordinate System, Archimedean Spiral, Fermat's Spiral
1. ↑ Weisstein, Eric W. (2011). http://mathworld.wolfram.com/Limacon.html. Wolfram:MathWorld.
2. ↑ 2.0 2.1 Stewert, James. (2009). Calculus Early Transcendentals. Ohio:Cengage Learning.

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