下周我们从变量变化的观点，来看证明这个公式。
We will also see another proof of this formula, using changes of variables, next week.
现在还是皆大欢喜，对变量做一点变化。
OK, so far we are pretty happy. Now let's do a change of variables.
若是变量的类型名称较长，或是会经常变化，那么这样的声明方法更是将显得非常乏味。
When the type names are long or are changed frequently, this can become quite tedious.
这些变量表示会随时间而变化的值。
These variables represent a value that can change over time.
无法导致在状态或输出变量上的变化。
如何改变一个视图的帧时，一个变量的值的变化？
How to change the frame of a view when a variable value changes?
价值指数用以度量某些变量的价值总量变化。
A value index measures general changes in the total value of some variable.
永远记得考虑变量的变化。
并得出了损伤变量分别随应力和应变而变化的规律。
The evolutional law of damage variable as a function of stress was obtained.
实际上，环境因子应该是随时间变化的变量。
Actually, the environmental factor ought to be varying with testing time.
实际上，环境因子应该是随时间变化的变量。
Actually, the environmental factor ought to be varying with testing time.
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