How do you say "Hi. Whats up?" in Japanese. "Whats up" is used when your freind called you for a reason...

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何かあった？、何があったの？、どうしたの？、どうかしましたか？、何でしょう、変わりはない？
元気？、どうしてる？、最近調子どう？

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If it's the "what's up?" that means "what's the matter?" it could be "どうしたの？"

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I think 'げんき？' or 'ちょうし　どう？' are better.
Your friend may say 'げんき' (I'm fine.) or  'まぁ　まぁ' (so so) or ' とくに' (nothing special) or something about yesterday etc...

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