# Homework Help: Finding Volume between Two planes Help

1. Mar 11, 2007

### CaityAnn

Finding Volume between Two planes "Help"

Ok heres the question
Find the volume of the region between places x+y+2z=2 and 2x+2y+z=4 in THE FIRST QUADRANT, using rectangular coordinates.

What I have done:

Graphed the planes. Created x=o y=o and z=o planes to remain in first quadrant for my own visuals. I set both the planes equal to zero. Solved for my x y and z intercepts. x=2, y=2 for both and z=1 for one and z=4 for another.
Now the set up my integrals. I need to integrate the z to go between the two planes so I was thinking to subtract one from the other..? That or I make me lower limit one of the planes and the other my upper limit. This is whats hanging me up... whether to subtract or not. If it do then its not the same function , but if I dont then I think my limits will have no "limit" to go from z=0 to another limit.

Help

2. Mar 11, 2007

### CaityAnn

Everyones lookin and nobodys saying anything. :/

3. Mar 11, 2007

### Tom Mattson

Staff Emeritus
One reason for that could be that this isn't the Homework Help section. Allow me to move your thread for you.

*kick*

Ah, there we are.

You mean first octant, don't you? This is 3-space, not 2-space.

Go with that. Find the volume between each given plane and the xy-plane and subtract the results. You'll need to do a double integral in both cases, but you can treat them as 2 separate mini-problems.

4. Mar 11, 2007

### CaityAnn

Im supposed to do a triple integral. Are you saying I should do for dz a integral from O to lower plane - a integral from o to highest... Should I compute for y and x for each of those integrals seperately and subtract two different answers?

5. Mar 11, 2007

### CaityAnn

well i get the same answer taking the integral of the lower plane completely and integral of upper plane completely, setting my dy=2-x both times, subtracting two answer= 2.

then i did a integral subtracting both the planes, y=2-x, x=0..2 and got 2 again.

Last edited: Mar 11, 2007
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