来自《耶鲁大学开放课程：基础物理 第16集 第16集》

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  2x cancels the 2, so I get f''(0)·x.
  约去 2 得到 f''(0)·x
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  But now, evaluate the derivative at x=0.
  现在求零点处的导数
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  That gets rid of this guy,
  这一项变为零
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  and the derivative of my test function
  近似函数的导数
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  matches the derivative of the actual function,
  跟实际函数的导数值一致
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  because this is the actual derivative of the actual function.
  因为这是实际函数的实际导数值
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  Now, it follows that if you want to say,
  接下来你可能会说
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  "How about the second derivative of this function?"
  "这个函数的二阶导数呢"
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  Let's take the test function I have, or the approximation.
  让我们试试的近似函数
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  Take a second derivative, and make sure that comes out right.
  来求二阶导数 以确保你的逼近是正确的
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  I don't feel like writing this out, but try to do this in your head.
  我不想写出来 但你可以试试心算一下
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  Take two derivatives on the left-hand side
  对左边求二阶导数