The mean displacement in each step is the average of these two values, which is:
$\bar{x}_{1} = \frac{(l-b) + (l+b)}{2} = l$
Since the particle takes $N$ steps, the total mean displacement is:
$\bar{x} = N \times \bar{x}_{1} = Nl$
(b) To find the dispersion,
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