# volume of solid inside ellipsoid

• Dec 6th 2009, 02:29 AM
pepsi
volume of solid inside ellipsoid
hello,

i want to find the mass of hte solid inside the ellipsoid

[(x^2)/(a^2)]+[(y^2)/(b^2)]+[(z^2)/(c^2)]=1, if the density is |xyz|

I make the change of variables x=ax', y=by', z=cz'
so that (x'^2)+(y'^2)+(z'^2)=1

so now i want

M = abc triple integral over the volume of the sphere of |abcx'y'z'|dx'dy'dz'

I change to cylindrical coordinates

so now i want

M = abc triple integral over teh volume of the sphere of |abcz'(r^2)sintcost|rdtdrdz'

because of the aboslute value in the integral i consider the mass in one octant and then multiply by 8 the relevant limits are

for the variable r, from 0 to 1
for the variable t, from 0 to pi/2
for the variable z', from 0 to 1

i end up with [(abc)^2]/2 but book says [(abc)^2]/6

please help i'm so v. frustrated with these cylindrical coordinates where i'm i messing things up? (i guess in the limits, but where?)
• Dec 6th 2009, 02:40 AM
pepsi
p.s.

thank anyway but think i've got it

the limits for r are instead: from 0 to squrt root of [1-(z'^2)]
(Happy)