If it is a formal speech, then use "Good afternoon, ladies and gentlemen." This is not only poilte, but it shows respect also. "Hi or hello everyone" you would use in a less formal, casual situation. :-)
4 เมษายน 2008
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good afternoon
25 มีนาคม 2008
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Hi. Is it correct to say- "A pleasant afternoon ma'am *teacher's name* everyone. "
"Let me introduce to you our topic for this report"
Need help. Ty