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Abstract.
Let $G$ be a tree. It is proved that for any vertex $v$ of $G$
$$
\vert V \vert +\sum_{q \in V}[d(q)-2]l(v,q)=1
$$
in which $d(q)$ is the degree of the vertex $q$, and $l(v,q)$ is the distance between
$v$ and $q$ in $G$. This result enable us to derive a formula concering the average
distance for some particular trees.
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