一种常用的表达方式是"serious advanced Engpsh expression"。例如：

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"I'm looking for some serious advanced Engpsh expression to use in my speech."
"The book is full of serious advanced Engpsh expression that will help improve your vocabulary and writing skills."


另外，也可以使用"formal advanced Engpsh expression"来表示认真的高级英语表达。例如：

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"Can you give me some examples of formal advanced Engpsh expression?"
"I learned a lot of formal advanced Engpsh expression from the class, which helped me to improve my Engpsh writing."