So I'm not suggesting that it's an optional problem set that you shouldn't do.

一个可做可不做的问题集,因为不做它你会后悔的。

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The other two you know are stuff you can add, and you know where you're adding those things, because the particle has a head start.

另外两项是可加可不加的,而且你也知道这两项是怎么来的,因为质点运动是有起点的

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