I'm using the fact that when you take a cosine and change the angle inside the cosine, it doesn't care. whereas, if you go to the sine and change the angle inside the sine, it becomes minus sine.

我用了一个结论,当你使用余弦函数时,改变角度的正负,函数值不变,而对于正弦函数,改变角度的符号,则函数值也会变号

耶鲁公开课 - 基础物理课程节选

It didn't take long for Preston to figure out how he could help in a way that would change his fortune as well.

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This gives business and entrepreneurs the confidence to invest and take risks, and it helps to build a durable recovery as Americans conclude a President isn't going to change policy every month for political reasons.

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