The coefficient of a root in a root system must be 0 or at least 1

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Let W be a Coxeter group (not necessarily finite), and let Π and Φ be the corresponding root basis and root system. Suppose that x ∈ Φ + and a ∈ Π such that the coefficient of a in x is not zero. Prove that this coefficient must be greater than or equal to 1. You may assume that Π is linearly independent.