If R is a rectangle
in the x-y plane and f(x,y) is a function defined on R
then we saw that ∬Rf(x,y)dA is what we get
when we
Chop R into a bunch of small boxes.
Compute f(x∗,y∗)ΔxΔy
for each box contained in R.
Add up the boxes, and
Take a limit as we chop R into smaller and smaller
pieces.
The same ideas apply when R is an
arbitrary region that is not a rectangle. As
with rectangles, things get a lot simpler if we arrange the boxes
intelligently. This video explains the concepts.
The video above explained why we get the following summary of
integrals over an arbitrary region. Remember, g and h are functions on the xy-plane,
while f(x,y) is a surface over (or
under) the xy-plane.
Type I regions
are regions that are bounded by vertical lines x=a and
x=b, and curves y=g(x) and y=h(x), where we assume
that g(x)<h(x) and a<b. Then we can integrate
first over y and then over x:∬Rf(x,y)dA=∫bx=a∫h(x)y=g(x)f(x,y)dydx
instruct.math.lsa.umich.edu
Type II regions
are bounded by horizontal lines y=c and y=d, and curves
x=g(y) and x=h(y), where we assume that g(y)<h(y)
and c<d. Then we can integrate first over x and
then over y:∬Rf(x,y)dA=∫dy=c∫h(y)x=g(y)f(x,y)dxdy