Problem on Root Systems

Question

Let $R$ be a root system on a Euclidean space $\mathbb{E}$ with simple roots $\{\alpha_i : i=1,....,l\}$. If $\alpha$ = $\sum_{i=1}^{l}c_i\alpha_i$ be a root , then show that $\frac {c_i(\alpha_i|\alpha_i)}{(\alpha|\alpha)}$ $\in$ $\mathbb{Z}$ $\forall$ $i$ = $1, 2....,l$. Any help will be highly appreciated.