# Polar Equations

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## Revision as of 16:51, 15 July 2011

A polar rose (Rhodonea Curve)
This polar rose is created with the polar equation: $r = cos(\pi\theta)$.

# 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

## Rose

The general polar equations form to create a rose is UNIQ6cc944302ab5de13-math-00000001-Q [...]

## 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 maximum 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 $\pi$.
Examples:
• If $n$ is an even integer, then there would be $2n$ petals, and the curve repeats itself every $2 \pi$.
Examples:
• 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.
Examples:
 $r = \cos(\frac{1}{2}\theta)$ The angle coefficient is $\frac{1}{2} = 0.5$. $1 \times 2 = 2$, which is even. Therefore, the curve repeats itself every $\pi \times 2 \times 2 \approx 12.566.$ $r = \cos(\frac{1}{3}\theta)$ The angle coefficient is $\frac{1}{3} \approx 0.33333$. $1 \times 3 = 3$, which is odd. Therefore, the curve repeats itself every $\pi \times 3 \times 1 \approx 9.425.$
• If $n$ is irrational, then there are an infinite number of petals.
Examples:
$r = \cos(e \theta)$
$\theta \text{ from } 0 \text{ to...}$
 ...$10$ ...$50$ ...$100$

$\text{Note: }e \approx 2.71828$

Below is an applet to graph polar roses, which is used to graph the examples above:

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

Source code: Rose graphing 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 $b = a/2$, then it is a trisectrix (see figure 2).
• If $b = a$, then it becomes a cardioid (see figure 3).
• If $2a > b > a$, then it is dimpled (see figure 4).
• If $b \geq 2a$, then the curve is convex (see figure 5).
 $r = \cos(\theta)$ 1 $r = 0.5 + \cos(\theta)$ 2 Cardioid $r = 1 + \cos(\theta)$3 $r = 1.5 + \cos(\theta)$4 $r = 2 + \cos(\theta)$5

## Finding Derivatives[2]

A derivative gives the slope of any point in a function.
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:
$\frac{dy}{dx} = \frac{\frac{dy}{d\theta}}{\frac{dx}{d\theta}} = \frac{\frac{dr}{d\theta} \sin(\theta) + r \cos(\theta)}{\frac{dr}{d\theta} \cos(\theta) - r \sin(\theta)}$

Note: It is not necessary to turn the polar equation to parametric equations to find derivatives. You can simply use the formula above.
Examples:

Find the derivative of $r = 1 + \sin(\theta)$ at $\theta = \frac{\pi}{3}$.
$\frac{dr}{d\theta} = \cos(\theta)$

$\frac{dy}{dx} = \frac{\frac{dr}{d\theta} \sin(\theta) + r \cos(\theta)}{\frac{dr}{d\theta} \cos(\theta) - r \sin(\theta)} = \frac{\cos(\theta) \sin(\theta) + (1 + \sin(\theta) ) \cos(\theta)}{\cos(\theta)\cos(\theta) - (1 + \sin(\theta) ) \sin(\theta)}$

$= \frac{\cos(\theta)\sin(\theta) + \cos(\theta) + \cos(\theta)\sin(\theta)}{\cos^2(\theta) - \sin(\theta) - \sin^2(\theta)}$

Note: Using the double-angle formula, we get $\cos^2(\theta) - \sin^2(\theta) = 1 - 2\sin^2(\theta)$

$= \frac{\cos(1+2\sin(\theta))}{1-2\sin^2(\theta)-\sin(\theta)} = \frac{\cos(1+2\sin(\theta))}{(1+\sin(\theta))(1-2\sin(\theta))}$

$\frac{dy}{dx} \Big |_{\theta=\pi/3} = \frac{\cos(\pi/3)(1+2\sin(\pi/3))}{(1+\sin(\pi/3))(1-2\sin(\pi/3))}$

$= \frac{\frac{1}{2}(1+\sqrt{3})}{(1+\sqrt{3}/2)(1-\sqrt{3})} = \frac{1+\sqrt{3}}{(2+\sqrt{3})(1-\sqrt{3})} = \frac{1+\sqrt{3}}{-1-\sqrt{3}} = -1$

## Finding Areas and Arc Lengths[2]

Area of a sector of a circle.
To find the area of a sector of a circle, where $r$ is the radius, you would use $A = \frac{1}{2} r^2 \theta$.
$A = \int_{-\frac{\pi}{4}}^\frac{\pi}{4}\! \frac{1}{2} \cos^2(2\theta) d\theta$

Therefore, for $r = f(\theta)$, the formula for the area of a polar region is:

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

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

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

# Why It's Interesting

$r = \sin^2(1.2\theta) + \cos^3(6\theta)$
The disc phyllotaxis of a sunflower.[3]

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 the Cardioid.

Archimedes' spiral can be used for compass and straightedge division of an angle into $n$ parts and circle squaring. [4] Fermat's spiral is a Archimedean spiral that is observed in nature. The pattern happens to appear in the mesh of mature disc phyllotaxis. Archimedean spirals can also be seen in patterns of solar wind and Catherine's wheel.

As you can see, these equations can create interesting curves and patterns. More complicated patterns can be created with more complicated equations, like the image on the right. Since intriguing patterns can be expressed mathematically, like these curves, they are often used for art and design.

## 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.

# About the Creator of this Image

Polar Coordinates
Cardioid
Source code: Rose graphing applet

# 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.
3. Lauer, Christoph. http://www.christoph-lauer.de/Homepage/Blog/Eintrage/2009/10/6_Fermats_Spiral_Suflower_Generator.html. Christoph Lauer's Blog.
4. Weisstein, Eric W. (2011). http://mathworld.wolfram.com/ArchimedesSpiral.html. Wolfram:MathWorld.