Because when isothermal, volume change+pressure change involves work that cannot be calculated by high school mathematics knowledge. . . . Moreover, the isothermal expansion or compression of workmanship actually involves some knowledge about heat engines, which was not learned in high school. . .

Because there are only a few quantities that determine the state of gas, such as molecular weight, temperature, volume and pressure, any three quantities are certain, and the remaining quantities are certain, with only one certain value.

There will be no phenomenon that a certain volume of gas may have two pressures at one temperature.