Answer
$d=(\frac{L}{\theta})(1-\cos \frac{\theta}{2})$
Work Step by Step
Substituting the answer from 77, $r=\frac{L}{\theta}$, into the answer of 79:
$d=r(1-\cos \frac{\theta}{2})$
$d=(\frac{L}{\theta})(1-\cos \frac{\theta}{2})$
This is the formula that gives $d$ for specific values of $\theta$.