I want to write down for you equations for a moving particle a moving object in three-dimensional space very complicated motion which I can hardly imagine what it's like.
我下面给大家，粒子的运动方程，即在三维空间中物体的运动，非常复杂的运动，很难想象它是如何的。
I very seldom write letters. But in this moving occasion I couldn't help myself.
我非常少写信，但这会儿我觉得一定要写点什么。
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