Answer
$r =\ \cos(\theta)$
Work Step by Step
The conversion of polar co-ordinates $(r, \theta)$ to rectangular coordinates $(x,y)$ can be expressed as:
$x=r \ \cos(\theta)$, $y=r \ \sin(\theta) $
where, $r=\sqrt{x^2+y^2}$
We have : $x^2+y^2=x$
This implies that $r^2= r \ \cos(\theta)$
Therefore, $r =\ \cos(\theta)$
