From 1983 to the present as you just said, the economy has a long period of smooth, robust expansion interrupted by two very mild recessions.

第二时期是从1983年至今，正如你所说，经济长时间处于,平稳强健的发展状态,而只出现了两次轻度衰退。

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That means, when I take two derivatives, I want to get a, then you should know enough calculus to know it has to be something like at^, and half comes from taking two derivatives.

也就是说,当我想求二阶导时,得到了a,你应该有足够的微积分知识,才能知道必须有类似at^的项,而这个1/2则是因为求了两次导数

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