ok so we got the area between those to curves
and x=y between them
i cant understand this transition
why you change the area from R1 to R2 ,on what basis
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