## Putting It Together: Parametric Equations and Polar Coordinates

### Describing a Spiral

Recall the chambered nautilus introduced in the chapter opener. This creature displays a spiral when half the outer shell is cut away. It is possible to describe a spiral using rectangular coordinates. Figure 1 below shows a spiral in rectangular coordinates. How can we describe this curve mathematically?

Solution:

As the point P travels around the spiral in a counterclockwise direction, its distance d from the origin increases. Assume that the distance d is a constant multiple k of the angle $\theta$ that the line segment OP makes with the positive x-axis. Therefore $d\left(P,O\right)=k\theta$, where $O$ is the origin. Now use the distance formula and some trigonometry:

$\begin{array}{ccc}\hfill d\left(P,O\right)& =\hfill & k\theta \hfill \\ \hfill \sqrt{{\left(x - 0\right)}^{2}+{\left(y - 0\right)}^{2}}& =\hfill & k\text{arctan}\left(\frac{y}{x}\right)\hfill \\ \hfill \sqrt{{x}^{2}+{y}^{2}}& =\hfill & k\text{arctan}\left(\frac{y}{x}\right)\hfill \\ \hfill \text{arctan}\left(\frac{y}{x}\right)& =\hfill & \frac{\sqrt{{x}^{2}+{y}^{2}}}{k}\hfill \\ \hfill y& =\hfill & x\tan\left(\frac{\sqrt{{x}^{2}+{y}^{2}}}{k}\right).\hfill \end{array}$

Although this equation describes the spiral, it is not possible to solve it directly for either x or y. However, if we use polar coordinates, the equation becomes much simpler. In particular, $d\left(P,O\right)=r$, and $\theta$ is the second coordinate. Therefore the equation for the spiral becomes $r=k\theta$. Note that when $\theta =0$ we also have $r=0$, so the spiral emanates from the origin. We can remove this restriction by adding a constant to the equation. Then the equation for the spiral becomes $r=a+k\theta$ for arbitrary constants $a$ and $k$. This is referred to as an Archimedean spiral, after the Greek mathematician Archimedes.

Another type of spiral is the logarithmic spiral, described by the function $r=a\cdot {b}^{\theta }$. A graph of the function $r=1.2\left({1.25}^{\theta }\right)$ is given in Figure 2. This spiral describes the shell shape of the chambered nautilus.