I'm going to do something which somehow we are told never, ever to do, which is to just cancel the dts.

我接下来要做一件以前绝对不允许的事情,约去这些dt

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So, the point is whenever you have derivatives with something over dt, do not hesitate to cancel the dts This will be actually true as long as both quantities are vanishingly small.

关键在于,只要你推出的式子分母上有dt,不要犹豫,直接约掉,这个式子只在,两遍都是无穷小的情况下成立

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