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Here is exactly what is stated on the theory page of my book:

Example: Area of a Region

Thearea of a region R in the xy-planecorresponds to the case wheref(x,y)=1.

Area of R= ∫∫dR

Example: Mass of a Region

Themass of a region R in the xy-planewith 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!

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# Vector Calculus: Area and Mass of a Region

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