来自《耶鲁大学开放课程：新生有机化学 第3集 第3集》

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  to realize that that's the way you'd like to hike,
  知道这才是你会想要选择的路径
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  or that if you spilled a cup of water up at the pass
  或者如果你将一杯水倒在这分水岭上
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  it would run down both ways.
  它会从两侧都流下
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  Now in fact
  事实上
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  we can use that concept to make it even a simpler graph,
  我们可以用那个概念画一个更简单的图形
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  where we don't need to draw contour lines,
  这样我们就不需画出等高线
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  because we could take the steepest descent path--
  因为我们可以走最陡的下降路径
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  if you pour the water out at the pass
  如果你将水倒在分水岭上
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  and follow how it'll trickle down,
  并看清它流下的方向
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  it'll take the steepest way to go down.
  会发现它采取速度最快的下降路径流下
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  It turns out it crosses every contour perpendicularly.
  结果是它垂直穿过了每一条等高线
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  So you follow that
  所以你跟着它