Linear equation means that solutions are additive.

线性方程的结果是可加的。

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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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We are able to talk about the fact that two plus one equals three, but it's not as though we ever come across numbers-- number three itself--anywhere in the empirical world.

我们可与去探讨,二加一等于三的这个事实,但是我们不可能有一天真的碰到数字,在现实世界中碰到个叫"数字三"的东西

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