In my book the meaning of как дела? is given as how are you?
Also the meaning of как у тебя дела? is given as how are you?
So which one should I use?
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3No much difference, the latter is just a bit more personal. Also keep in mind that in Russian both phrases are more about truly asking the question than in English, where "how are you?" is usually just a meaningless exchange of niceties. – Headcrab Nov 9 '17 at 8:17
"Как дела?" can be roughly translated as "How are things?"
"Как у тебя дела?" -> "How are things with you?"
So one is just a shorter version of the other.
There is also the polite version: "Как у вас дела?". For more background on the тебя/вас distinction, see this question.
'как дела?' is more like a polite way to start a conversation, compare to:
how are you doing? fine, and you? i'm ok.
'как у тебя дела?' shows more interest in the actual state of things of a person you have a conversation with. The difference is subtle, yet I think it exists.
