How can the area recovery of a surface be automated without a macro?

Hello

    I'm currently stuck on a problem that seems quite silly on paper.

I try to retrieve the surface of a selected surface such as "face<1>" in order to be able to add them automatically but I can't find the corresponding syntax.

I manage to recover the total surface area of the room with: "SW-Area" or  "SW-Superficie@Bordure circular<1>. Part" which allows me to return the total area of the selected room

But I can't isolate one face.

I don't want an alternative solution because I've already found some.

But I am trying to successfully recover this value via the equations 

Thank you for your help =)