- #1
JonnyG
- 233
- 30
Homework Statement
Let [itex]S[/itex] be the tetrahedron in [itex]\mathbb{R}^3[/itex] having vertices [itex](0,0,0), (1,2,3), (0,1,2), [/itex] and [itex] (-1,1,1)[/itex]. Evaluate [itex]\int_S f[/itex] where [itex]f(x,y,z) = x + 2y - z[/itex].
Homework Equations
The Attempt at a Solution
I just want to confirm that I am setting up the integral properly: Looking at the projection of the tetrahedron onto the [itex]xy[/itex]-plane, it looks like [itex]-1 \leq x \leq 1[/itex] and [itex]-x \leq y \leq 2x[/itex]. Now looking at the actual tetrahedron, it seems as if [itex]z[/itex] varies between [itex]0[/itex] and the plane [itex]-x + z - 2 = 0[/itex] so that the boundaries for [itex]z[/itex] are: [itex] 0 \leq z \leq x + 2[/itex]. Therefore [itex]\int_S f = \int_0^{x+2} \int_{-x}^{2x} \int_{-1}^1 f \text{ } dxdydz[/itex]. Is this correct?
EDIT: Wait, this makes no sense. If that is my setup, then my final integral will have an [itex] x [/itex] in it. Forget the projection onto the xy-plane. Looking at the tetrahedron, it looks as if [itex] x [/itex] is bounded between the two planes [itex] -x + 2y - z = 0 [/itex] and [itex] x + 4y - 3z = 0 [/itex] so that [itex] 2y - z \leq x \leq 3z - 4y [/itex]. It seems as if [itex] y [/itex] is bounded between the two planes [itex] x + 2y - z = 0 [/itex] and [itex] y = 2 [/itex] so that [itex] \frac{z}{2} - \frac{x}{2} \leq y \leq 2 [/itex]. It looks as if [itex] z [/itex] is bounded between the xy plane and [/itex] -x + z - 2 = 0 [/itex] so that [itex] 0 \leq z \leq x + 2 [/itex]. So that my integral should be [itex] \int_0^{x+2} \int_{\frac{z}{2} - \frac{x}{2}}^2 \int_{2y - z}^{3z - 4y} f \text{ } dxdydz [/itex]. Is this correct?
Also, the book gives a hint: Find a suitable linear transformation [itex] g [/itex] as a change of variables. I've been trying to find a linear diffeomorphism from the tetrahedron to the unit cube (or a diffeomorphism from a set that differs from the tetrahedron by measure zero to a set that differs from the unit cube by measure zero), but have been unable to find such a mapping. However, perhaps that is the easier route than to try and do what I am currently doing?
Last edited: