Hi y'all. Here is exactly what is stated on the theory page of my book: Example: Area of a Region The area of a region R in the xy-plane corresponds to the case where f(x,y)=1. Area of R= ∫∫dR Example: Mass of a Region The mass of a region R in the xy-plane with mass density per unit area ρ(x,y) is given by: Mass of R= ∫∫ρdR I'm not at all understanding this first part of the theory, why is it that the area of the region R in the xy-plane is the case f(x,y)=1 and how did they obtain that express for the area R? All help is immensely appreciated as I'm tearing my hair out over this. Thanks!