Daily Equation #5 - Fermat's Spiral

This is Day 5 of my collection of printable equations. To see the whole collection, go to: http://www.thingiverse.com/thing:227210

For today's equation, I'm revisiting the Archimede's spiral (r = a*theta) of day #3 (http://www.thingiverse.com/thing:228425). A variant is Fermat's spiral, r = +/-a*theta^0.5 (http://en.wikipedia.org/wiki/Fermat%27s_spiral). In contrast to Archimede's, Fermat's spiral exhibits a flat region near theta = 0 and a continuously decreasing distance between turns. It a