# Homework Help: Lagrange Multipliers, calc max volume of box

1. Nov 18, 2008

### hils0005

1. The problem statement, all variables and given/known data

Point P(x,y,z) lies on the part of the ellipsoid 2x^2 + 10y^2 + 5z^2 = 80 that is in the first octant of space. It is also a vertex of a rectangular parallelpiped each of whose sides are parallel to a coordinate plane. Use Method of LaGrange Multipliers to determine the coordinates of P so that the box has a max volume and calculate the max

2. Relevant equations
f(x,y,z)=xyz g(x,y,z)=2x^2+10y^2+5z^2=80

3. The attempt at a solution

$$\nabla$$f=$$\nabla$$g$$\lambda$$

1.yz=4x$$\lambda$$
2.xz=20y$$\lambda$$
3.xy=10z$$\lambda$$

I multiplied equation 1 by x, 2 by y and 3 by z

4x^2$$\lambda$$=20y^2$$\lambda$$=10z^2$$\lambda$$

I then put x and z in terms of y and put into constraint
4x^2=20y^2 10z^2=20y^2
x=$$\sqrt{}$$5 y z=$$\sqrt{}$$2 y

g=2($$\sqrt{}$$5y)^2 +10y^2 + 5($$\sqrt{}$$2y)^2=80
solving for y=$$\sqrt{}$$(8/3)

I'm not sure if I'm on the right track or if this is way off, if correct do I just do the same proceedure to find x and z?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 19, 2008

### Dick

You are doing fine. You don't need to repeat the procedure for x and z. You already have x=sqrt(5)*y and z=sqrt(2)*y. Once you've got y, you've got everything.