## Calculus (3rd Edition)

The conic section is an ellipse and the directrix is $x=8$.
Converting the given equation to the standard form $$r=\frac{e d}{1+e \cos \theta}.$$ We get $$r=\frac{8}{4+ \cos \theta}=\frac{2}{1+(1/4) \cos \theta}.$$ Thus $e=\frac{1}{4}.$ Since $e\lt 1$ then the conic section is an ellipse. To find the directrix, we find $d$ first. Since $ed=2$ and $e=1/4$, then $d=8$ and hence the directrix is $x=8$.