Can you do this in your head?

你能心算出来吗
If this Δx-u·Δt,

如果这是 Δx-u·Δt
this denominator cancels between dividing this by this,

用这个除以这个约去分母
so it looks like--

就是这样的
I just divided this guy by this guy,

我只是用这一项除以这一项
because that's when you divide the left-hand side by the left-hand side,

因为当你用左边除以左边时
you have to divide the right-hand side by the right-hand side.

你也得用右边除以右边
Now, you should have an idea of what I'm planning to do.

你们应该知道我要干什么了
What do we do next to this expression here?

这个式子我们接下来该怎么处理
Yes?

请讲
A limit of what?

什么的极限
If we just take a limit of Δt going to 0,

如果我们只是取趋于零的 Δt 的极限