Solution (source code)

= Solution

For the planar <random walk> with independent steps of length $b$, cross terms vanish and $\langle R^2\rangle=Nb^2$. Thus $R_{\rm rms}=bN^{1/2}$. At large $N$, the planar Gaussian end distribution gives $\langle R\rangle\simeq b\sqrt{\pi N}/2$.