If you want to book a journey, or a flight, you can say: I'd like to book a trip to Xi 'an.

如果你想预订一个旅程或者飞机票，你可以说：Id like to book a trip to Xian。

youdao

You should visit Xi 'an if you have time.

如果你有时间应该到西安看看。

youdao

If one day you come to Xi 'an, you'd better visit some places of interest, so that you can learn more knowledges about this city.

如果有一天你来到西安，你最好参观一些名胜古迹，这样你可以学习更多关于这个城市的知识。

youdao

If you have a sample with n observations, it's the summation I = 1 to n of xi/n--that's the average.

如果你有n个观测值,对Xi从i=1到n求和再除以n

耶鲁公开课 - 金融市场课程节选