1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Change Order of integration

  1. May 7, 2014 #1
    1. The problem statement, all variables and given/known
    Show that

    ∫∫∫ 12y^2 z^3 sin[x^4] dxdydz

    Region: { y< x< z
    0< y< z
    0 <z< (Pi)^ 1/4

    Equals Pi/4
    Change order of integration to dydxdz

    2. Relevant equations

    Order of integration
    3. The attempt at a solution

    First I graphed my region on the on the zy- plane and then I graphed my region on the xz- plane . I know my limits of integration for z will not change bc I'm still going from 0 to (Pi)^1/4

    And as far as my limits for dy I have to put it in terms of x and z.
    But I'm not seeing what my limits are to convert to dydxdz. ImageUploadedByPhysics Forums1399499243.358124.jpg
  2. jcsd
  3. May 7, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Limits are given in the formulation of the exercise. They don't change. So $$\int_0^z \, 12 y^2 dy$$ is your first to work out.
  4. May 7, 2014 #3

    We are instructed to change the order to dydxdz that's why.
  5. May 7, 2014 #4
    Think of the z as a constant. And draw a graph of your limits as you would normally do for an area of integration in two variables in the x-y plane.

    Hint*: ##0\leq x\leq z, 0\leq y\leq## ? What is the line that bounds the shaded region between the y axis and y=x?
    Last edited: May 7, 2014
  6. May 7, 2014 #5

    I would think x=y

    So wrt dx limit of integration is from 0 to y?
  7. May 7, 2014 #6
    Sorry I mixed up the x and y in my intervals earlier. The dx limit is from 0 to z and the dy limit is from 0 to x. Tell me if you can understand why this is so. Notice that the dx limit is clearly bounded by 0 and z based on your diagram, and the shaded area has to be bounded by 0 and the line y=x.
    Last edited: May 8, 2014
  8. May 8, 2014 #7

    Ohh ok good that's how I originally had it set but wasn't sure if it was correct.... But once integrated it equals pi/4 . So it works.
    Referring to z as a constant really helped out to graph the xy-plane thank you!:)
  9. May 9, 2014 #8


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Yes. Meaning: integrate over dy, then over dx and finally over dz:
    $$\int_0^{\root 4 \of \pi} \,\int_0^z \,\int_y^z \, 12 y^2 z^3 \sin x^4 \ dx dy dz = \int_0^{\root 4 \of \pi} \,\int_y^z \,\int_0^z \, 12 y^2 z^3 \sin x^4 \ dy dx dz = $$
    $$ \ \ \ \int_0^{\root 4 \of \pi} \, z^3 \int_0^z \, \sin x^4\, \left ( \int_y^z \, 12 y^2 \ dy \right ) dx dz $$ etc.
  10. May 9, 2014 #9


    User Avatar
    Homework Helper

    That's ... not how it works.
  11. May 9, 2014 #10
    This is totally wrong!
  12. May 9, 2014 #11
    This guy did a great job in explaining it for me, maybe he can help you too

    Last edited by a moderator: Sep 25, 2014
  13. May 9, 2014 #12


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    @yazz912: The only way to sensibly attack a problem like this is to draw the figure in three dimensions and use it to change the order of integration. That's what you would do in 2 dimensions, isn't it? So here's a hint for this problem. I will use ##h## instead of ##\sqrt[4]\pi##. On a carefully laid out xyz coordinate system mark the points ##(0,0,0),(h,h,h),(h,0,h),(0,0,h)##. Join these points to form a tetrahedron. See if you can convince yourself that this solid gives the region that is being described with your original triple integral. Once you satisfy yourself that that is true, then use the figure to do the integration in the other order.
  14. May 11, 2014 #13


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    My apologies to Jazz, total blooper missing out on x > y. PA and Dirk are of course absolutely right, albeit not very constructively. Feodo is more helpful and LC gives the best advice. My blind spot must have come from too much physics ("never a problem" :smile:) and not enough math (can be tricky :frown:).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted