来自《耶鲁大学开放课程：基础物理 第6集 第6集》

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  So, once you've got
  所以一旦你们了解了
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  the notion of a function of two variables,
  两个变量函数的概念
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  if you're going to do calculus, the next thing is
  如果想做微积分的话 接下来就要问
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  what about the derivatives of the function.
  函数的导数是怎样的呢
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  How does it change?
  它是怎样变化的
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  Well, in the old days, it was dependent on x
  用从前的观点来看 它取决于 x
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  and I changed the x and I found the change in the function
  我改变 x 的量 然后求出函数的变化量
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  divided by the change in x and took the limit
  除以 x 的变化量 然后取极限
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  and that became my derivative.
  就得到了要求的导数
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  And now, I'm sitting in the xy plane,
  现在我在 xy 平面上
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  so here is x and here is y;
  这是 x 这是 y
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  I'm here, the function is coming out of the blackboard.
  我在这 函数从黑板里出来