她坚信阶级制度恒定不变。

She believed in the fixity of the class system.

《柯林斯英汉双解大词典》

就假定保持a恒定不变。

Let's say keep a constant.

youdao

首先，a是恒定不变的。

Well, first I said a is constant.

youdao

The Joule experiment was a constant energy experiment, right. Here we're going to have to find a constant enthalpy experiment, and that is going to be the Joule-Thomson experiment. That's going to extract out a physical meaning to this derivative here.

非常像焦耳实验,焦耳实验是一个能量恒定的实验,我们这里要做的是，找到一个焓不变的实验,也就是焦耳-汤姆逊实验,这个实验可以把这里的微分式形象化。

麻省理工公开课 - 热力学与动力学课程节选

du/dT constant pressure is the direct derivative with respect to temperature here, which is sitting by itself under constant volume keeping this constant but there is temperature sitting right here too.

偏U偏T，p恒定是对,温度的直接微分，而它本身对体积不变,保持它不变，但是这里也有一个温度,这就是偏U偏V，T恒定。

麻省理工公开课 - 热力学与动力学课程节选