滑冰三周半可以用英文表示为 "skating for three and a half weeks". 这个表达来源于英文中表示时间长度的方法，即在数字后面加上时间单位，例如：

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- I have been working here for three years. 我在这里工作了三年。
- She has been studying Engpsh for six months. 她学习英语已有六个月。
- He has been playing the piano for two hours. 他弹钢琴已有两个小时。


此外，当数字大于1时，单位前需要加上冠词 "a" 或 "an"，例如：

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- I have been working here for a year. 我在这里工作了一年。
- She has been studying Engpsh for an hour. 她学习英语已有一小时。


所以，滑冰三周半可以用英文表示为 "skating for three and a half weeks"，中文翻译为 "滑冰三周半"。

另外，如果您想要在英文句子中表示这个概念，可以这样说：

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- I have been skating for three and a half weeks. 我滑冰三周半了。
- She has been skating for three and a half weeks. 她滑冰三周半了。
- He has been skating for three and a half weeks. 他滑冰三周半了。


希望这些信息能够帮到您！