So instead you'd have to maybe if you start with wavelength, go over there, and then figure out velocity and do something more like kinetic energy equals 1/2 n b squared to get there.

这时你要先从波长开始，到这，然后算出速度,然后像动能等于1/2nb平方得到这。

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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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