在开会是“at a meeting”还是“in a meeting”?最好有确定,又根据的出处

回答：
at a meeting和in a meeting都可以说,请看实例(大都出自权威词书)：
Officially,the director is in a meeting,though actually he’s playing golf.据说主任正在开会,可他实际上正打高尔夫球呢.——见《牛津高阶英汉双解词典》(第四版) (officially词条)
I’m tied up in a meeting until 3 pm.我开会直到下午三点钟方可脱身.——见《牛津高阶英汉双解词典》(第四版) (tie词条)
Sorry to keep you waiting—I got stuck in a meeting.——见《朗文当代英语词典》(1995年版) (keep词条)
Mrs Lavelle is in a meeting at the moment.——见《朗文当代英语词典》(1995年版) (meeting词条)
There were three others present at the meeting besides us.除了我们之外,还有其他三人个出席了会议.——见《朗文多功能分类词典》(besides词条)
He took the chair at a meeting some time ago.前些日子他主持了一次会议.——见王福祯编著《新时代英语惯用法词典》第195页.
There were both blacks and whites at the meeting.参加会议的有黑人也有白人.——见周贞雄编著《大学英语重点词用法手册》besides词条)
She might be invited to speak at the meeting.她可能被邀在会上发言.——见张道真编著《张道真英语语法》第116页.