来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  Meaning?
  也就是说
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  Oh, but this is nothing to do with dimension, you see.
  这可和维度无关 看这里
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  It's one number minus another number.
  只是一个数减去另一个数
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  I put another number in between and in this function;
  在函数中我放了另一个数在中间
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  it's a function only of, here.
  这个函数只是 这里
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  Let me see, x is not changing at all and y is changing,
  我看看 x 丝毫不变 而 y 在变化
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  so it is df / dy·Δy.
  所以是 df / dy·Δy.
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  I meant something else.
  我是指别的
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  That is, in fact, a good approximation
  事实上 这是个很好的近似值
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  but there is one thing you should be careful about
  但有一件事必须要注意
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  which has to do with where the derivatives are really taken.
  和求导的真正对象有关
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  For the term here,
  这个表达式