It is an important rule that we must combine theory with practice.

我们必须把理论与实践相结合是个重要的原则。

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So we must combine competition with cooperation to reach our goals.

所以，我们必须把竞争和合作结合起来达到(实现)我们的目标。

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That we must combine theory with practice is an important principle.

我们必须把理论与实践相结合，这是一个重要的原则。

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We can combine these two to see V2 that V4 over V1 must equal V3 over V2.

我们把这两个结合起来,得到V4除以V1等于V3除以。

麻省理工公开课 - 热力学与动力学课程节选