Isothermal pressure change

[Samantha Joseph 1F]

Why is it that the equation for finding the entropy of a system with an isothermal pressure change uses P1/P2 when temperature and volume are typically T2/T1 or V2/V1?

[Ju-Wei Wang 1I]

According to the ideal gas law, specifically Boyle's Law, pressure is inversely related to volume, so at constant temperature, V2/V1=P1/P2.

[Matthew 1C]

I think the reason P₁/P₂ is used is because it is inversely proportional to volume, which uses V₂/V₁. Also since both pressure and volume are directly proportional to temperature, does it matter which one uses 1/2 or 2/1?

[Ashley Macabasco 2K]

The change in pressure equation is P1/P2 opposed to P2/P1 like for the change in temperature and the change in volume because of Boyle's law in which volume and pressure are inversely proportional.