Answer
$r=\frac{1}{1+sin\theta}$.
Work Step by Step
1. Based on the given direction and position of the directrix, we know the equation has a form
$r=\frac{ep}{1+e\ sin\theta}$,
2. With $e=1$ and $p=1$, we have $r=\frac{1}{1+sin\theta}$.
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