To find the slope of the tangent to the curve $r = a / (1 + \cos \theta)$, we first need to find the rectangular coordinates $(x, y)$ in terms of $\theta$. We know that $x = r \cos \theta$ and $y = r \sin \theta$. So,
$x = \frac{a \cos \theta}{1 + \cos \theta}$
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