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The expressions "再见就是" and "你好就是" are commonly used in spoken Chinese, particularly in mainland China. They are informal ways of saying "goodbye" and "hello" respectively. The addition of "就是" at the end of these expressions adds emphasis and conveys a stronger meaning. It implies that the action of saying goodbye or hello is definitive and absolute. It can be seen as a way of saying "goodbye for real" or "hello for real". For example, if someone says "再见就是" when parting ways, it suggests that they are truly saying goodbye and there is no expectation of meeting again soon. Similarly, if someone says "你好就是" when greeting someone, it indicates a genuine and sincere hello.On the other hand, "再见" and "你好" are the standard and more formal ways of saying goodbye and hello respectively. They are used in both spoken and written Chinese in various contexts. These expressions are more neutral and do not carry the same level of emphasis as "再见就是" and "你好就是".To summarize, "再见就是" and "你好就是" are informal expressions that add emphasis to the act of saying goodbye and hello respectively, while "再见" and "你好" are the standard and more neutral ways of expressing these greetings.