ok so if we change the variables
is now
and
is now
this is R2
and in the integral of R2 the yacobian is -1
so the integral R1 equals minus the integral over R2
but why the sum of the integrals over R2 and R1 is the integral over R
?
because when we go from one coordinate systen to another out area is described in a totlay new axes
u ,v axes
so we cant do this thing
because R1 and R2 are on a diiferent axes system