Inactive Tutor answered 04/23/22
need region in xy plane to evaluate any double integral f(x,y)dxdy.
reason, say dx part evaluated first and ∫f(x,y)dx=F(x,y)+C(y)
C can be a constant or any function of y.
example C=sin(y)+ey*cos(y)+y4+8
that makes evaluating the integral [F(x,y)+C(y)dy part impossible
Inactive Tutor
it isn't that the C(y) used as example is impossible to integrate, it is that all C(y) functions are valid C(y), It is impossible to choose C(y).04/23/22