what's up?what's new,how are you doing?的区别和分别得回答

What's up?主要是来问“出了什么事?”也可以引申为你好吗?强调的是up,有什么事情发生.What's new?是问有什么新鲜事?强调的是new.而How are you doing?强调的是you.
What's up?你可以答.Nothing,everything is ok.或者可以告诉他出了什么事.What's new?你可以答：Nothing new.或者告诉他有什么新鲜事儿.对于How are you doing?你可以答：I am doing well.Thanks.