Answer
$\dfrac{2}{1+\cos \theta}$
Work Step by Step
The polar equation of a conic with eccentricity $e$ and directrix $x=k$ is defined as:
$r=\dfrac{ke}{1+e \cos \theta}$ ...(1)
We are given that the vertices are: $e=1,k=2$
Then $x=2$
Thus, equation (1), becomes
$r=\dfrac{(2)(1)}{1+(1) \cos \theta}=\dfrac{2}{1+\cos \theta}$
