12.You must have made the mistake, ______?

didn't you?

haven't u？

haven't you
反义疑问句，首先确定用否定，在确定 用助动词， 但是此句有两个 助动词 must 与have，但是 根据 语法规则，这里must have made 说话者对事件的推测，此类should/must/can/need+have+done的反义问句要根据情态动词后面的动词来变。

应该填haven't you.

这里must是加强语气的，整个句子是现在完成时，所以反义问句应该用haven't you.

正确答案应该是 haven't you
对已发生的过去情况的推测，若陈述句谓语部分有“must have done”，而且有表示过去的时间状语，问句部分用didn't；若没有表示过去的时间状语，问句部分用haven't或hasn't。如：
1)They must have gone there last night,didn't they?
2)They must have arrived by now,haven't they?（根据by now来判断）
3)They must have been to the Great Wall,haven't they?
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