# Centroid in three dimensions

## Homework Statement

Find the centroid of the region D, lying above the sphere x2+y2+z2 and below the paraboloid z=4-x2-y2.

## The Attempt at a Solution

I decided it might be easier to change it to polar coordinates soo I did
M=∫∫∫ dzrdrdθ
where z goes from sqrt(6-r2) to 4-r2
θ goes from 0 to 2∏
then i thought the r would be where the two functions intersect, so i set sqrt(6-r2)=4-r2 solving for r i got my solutions came out to be sqrt2 and sqrt5, i dont know which one to use because i thought r would go from 0 to either sqrt2 or sqrt5

if some one could help me out i would really appreciate it.. thanks
solving for r i got

## Answers and Replies

HallsofIvy
Science Advisor
Homework Helper
Look at a graph. $r^2+ z^2= 6$ (you forgot the "6" in your origina equation but it can be deduced from what you wrote later) and $z= 4- r^2$ intersect twice. For $r= \sqrt{2}$, $z= 4- 2= 2$ and for $r= \sqrt{5}$, $z= 4- 5= -1$. Because the problem says "above the sphere and below the paraoloid, you want to use the higher one, $r= \sqrt{2}$, z= 2.