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And I'm going to pause for a minute, before I run it, and ask each of you to compute in your head what you think the answer should be.
我将暂停一会儿，在我运行之前，我想问问你们你们,心算得出的答案应该是什么。
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Do this in your head and ask, "What can I possibly get?"
心算一下然后问问自己,"我能求出什么"
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Well, what I always do in anything like this is, the first thing I do is, I run it on something where I can actually compute the answer in my head. So I get a sense of whether or not I'm doing the right thing.
好的，我常常在,这种问题中会做的是，我会做的第一件事是我,会先带入我可以心算的值来运行,因此我能够知道我是否做对了。
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At least, not in your heads.
起码不用心算。
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The most important result from last time was that if you took this r, and you took two derivatives of this to find the acceleration, d^2 r over dt^2, try to do this in your head.
上节课最重要的结论,就是如果你把 r 写成这样,对 r 求两次导就能得到加速度,d^2 r / dt^2,心算一下
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