# Evaluate the triple integral

Evaluate triple integral bounded by a solid $S$ which is a tetrahedron defined by vertices $(0,0,0)$, $(0,0,1)$, $(1,1,0)$ and $(-1,1,0)$.

$$\iiint y^2\, dV$$

So, I tried to find the plane that contains the tree vertices except $(0,0,0)$ and with that I was planning to integrate the solid as I was doing with regions like that. But the plane doesn't have a coordinate x and so I'm in trouble at the integration. In the end, I have found the one set of limits:

$S= {{(x,y,z): 0<z<1 , 0< z <2-y , -y<x<-y}}$

• I have removed the image and LaTeX-ified this for you. We can use LaTeX on this site, and it makes life quite a bit easier than managing external links :) Jun 6, 2016 at 15:08

## 1 Answer

The plane that contains all the vertices except $0,0,1)$ is easy to find; it is the XY plane. Now if we slice the tetrahedron by a plane at $z = z_0$ where $0 \leq z_0 \leq 1$ that slice is a right triangle with vertices $$(0,0,0), (1-z_0,1-z_0,0), (z_0-1,1-z_0,0)$$ so the limits of integration to use to deal with such a plane is $$\int_{y=0}^{1-z} \int_{x=-y}^y dx \,dy$$ And the volume integral you want is $$\int_{z=0}^1\int_{y=0}^{1-z} \int_{x=-y}^y y^2\,dx \,dy \, dz$$ SPOILER ALERT Don't read on if you want to do the integral yourself. $$\int_{z=0}^1\int_{y=0}^{1-z} \int_{x=-y}^y y^2\,dx \,dy \, dz = \int_{z=0}^1\int_{y=0}^{1-z} \left[ xy^2 \right]_{x=-y}^{x=y} \,dy \, dz \\ = \int_{z=0}^1\int_{y=0}^{1-z} 2y^3\,dy \, dz = \int_{z=0}^1 \left[ \frac{y^4}{2}\right]_{y=0}^{1-z}\\ = \frac12\int_{z=0}^1 \left( 1-z \right)^4 \, dz=\frac{1}{10}$$

• Thank you Mark! I found the same thing that you, i was editing to include that information. Jun 6, 2016 at 15:56