来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  Or you might say, well, let me fix that by making it minus t;
  可能你会说 可以把式子调整为 - t
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  then it quickly comes to rest after a time and doesn't move.
  这样一段时间后它很快就会停下来
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  We sort of feel
  我们希望
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  we want this thing to be jiggling around for a long time.
  物体在较长的一段时间内一直来回振动
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  So, the exponential function is even better than this function
  实际上指数函数比这个要求的函数更好一些
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  because it says, take one derivative and I reproduce myself.
  因为指数函数的任意阶导数都是它本身
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  But that's too much;
  但是我们并不需要那样
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  we only want a function
  我们只需要一个函数
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  that reproduces itself when you take two derivatives,
  它的二阶导数是它本身即可
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  and that's where the trigonometric functions come in.
  这就是引入三角函数的目的
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  So, I make a guess, x is cos(t), and you can check
  假设 x(t) = cos(t) 你可以验证
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  if you take two derivatives, it obeys the equation,
  如果求二阶导数 它满足这个方程