来自《耶鲁大学开放课程：基础物理 第2集 第2集》

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  You will find then the result is Ax cos φ + Ay sin φ.
  你会发现结果就是 Ax ? cos φ + Ay ? sin φ
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  I'm using the fact that when you take a cosine
  我用了一个结论 当你使用余弦函数时
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  and change the angle inside the cosine,
  改变角度的正负
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  it doesn't care. whereas, if you go to the sine
  函数值不变 而对于正弦函数
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  and change the angle inside the sine, it becomes minus sine.
  改变角度的符号 则函数值也会变号
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  In other words, sine of minus θ is minus sin θ
  换句话说 sin( -θ ) = -sin θ
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  and cosine of minus θ is cos θ.
  而 cos( -θ ) = cos θ
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  This, by the way,
  顺便说一句
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  is another property I expect people to know in this course.
  这是这门课我想让大家了解的另一个性质
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  You cannot say,
  你就不能再说
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  "I never heard of it. I don't know where it came from."
  "我从没听说过 我不知道它是怎么来的"
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  You should think about why that is true.
  你应该想一想为什么那是正确的