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\sect{Average length of chain molecule}
Molecule can be described as an ${N}$ monomers chain.
The probability that a monomer will be horizontal is ${p}$ and then
it's length is ${a}$, otherwise it's length is ${b}$. Let ${L}$ be
the molecule's length.
Find ${f\left(n\right)\equiv
{p}\left(L=na+\left(N-n\right)b\right)}$. calculate
${\langle L\rangle}$ and
${\sigma_{L}}$, in two ways.\\
What is the behavior of ${\sigma_{L}/\langle L\rangle}$ as a
function of ${N}$?
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