Sorry if this is something really easy..
As said in the title, why when we want to compute the area or the volume there is something like this: $A(D_1) = \iint 1 \ dx\,dy\text{ or }\iiint 1 \, dx \, dy\ ?$
And when does this occur?
Is it a special case? In which, for example, for the area the one side of the parallelogram is "$1$" ?
Thank you for your time!