periodic functions, proof that lim x → + ∞ sin(x) does not exist

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I'm trying to figure out how to proof the next thing: Let f be a periodic function in real numbers,that is there exists T> 0 such that f (x) = f (x + T) ∀ x ∈ R. Show that if lim x → + ∞ f (x) exists, then f is a constant function. Deduce from this that lim x → + ∞ sin(x) does not exist.