The questions "How have you been?" and "What have you been up to?" can be used more or less interchangeably in an informal greeting between friends. Of course, asking "How are you?" is asking a broad question about the person (health, mood, etc.), while "Been up to much lately?" is asking a more specific question about what the person has been doing. Consequently, I might use "How are you?" with someone I haven't seen in a *long* time, because I want to ask a broad question about his overall status. On the other hand, I might use "Been up to much lately?" if I already know the general state of the person and want to know something more specific, i.e,. about his recent activities.

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