And that implies that since the quantity we want is given by this quantity, which is zero times a constant, the quantity we want is also zero.

因为我们需要的量,是由这个量乘以一个常数，因为这个量是零，因此我们需要的量也是零。

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What this really means is in a given time, the change in this quantity is a times the change in this quantity.

这里的真正含义是在给定的一段时间内,这个量的变化量等于,a乘以这个量的变化量

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