Find plane eqn that minimize Volume

In summary, the problem is to find the equation of the plane that minimizes the volume of a tetrahedron formed with the positive coordinate planes while passing through the point (1,1,1). This can be solved using the method of Lagrange multipliers, where the condition is that the gradients of the volume and plane equations are parallel. The obtained solution is a=b=c=3, which gives the plane equation x/3 + y/3 + z/3 = 1. The maximum volume of the tetrahedron cannot be obtained as it goes to infinity when one of the variables becomes large. If the axes are not the positive coordinate planes, the same method can be applied to find the equation of the new
  • #1
quietrain
655
2

Homework Statement


find plane that minimize Volume V if plane is constrain to pass through point 1,1,1

Homework Equations


plane equation = x/a +y/b + z/c = 1 , a,b,c > 0
with the positive coordinate planes and the plane, they form a tetrahedon of volume V = 1/6 abc

The Attempt at a Solution



how do i even start? :x
am i suppose to use the method of lagrangian multipliers?

thanks!
 
Physics news on Phys.org
  • #2
Lagrange multipliers sounds like a good approach to me. Why don't you just try it?
 
  • #3
er ok, so is this right?

i want to minimize V(a,b,c)=1/6 abc
my g(a,b,c) = x/a +y/b +z/c = 1

so ∇V = λ∇g ?
so Va = λga
1/6 bc = λ (-1/a2)

so for Vb and Vc, i get
1/6ac = λ(-1/b2)
1/6ab = λ(-1/c2)

so since the plane must pass through (1,1,1), i get g(a,b,c) = 1/a + 1/b +1/c = 1

so solving i get a=b=c=3?

so the plane is x/3 + y/3 +z/3 = 1 ?


but i have a few questions

1) shouldn't V be a function of x,y,z ? and not a,b,c? and so g should also be of x,y,z?

2) i didn't find the value of λ when i solve the simultaneous equations, is this ok? and even if i find it, what does it symbolize?

3) lastly, how do i know that the value a=b=c=3 gives me the minimized volume? why not it be the maximized?

thanks!
 
  • #4
quietrain said:
er ok, so is this right?

i want to minimize V(a,b,c)=1/6 abc
my g(a,b,c) = x/a +y/b +z/c = 1
No. Your condition on the plane x/a+ y/b+ z/c= 1 is that it contains the point (1,1,1).
Your condition is that g(a,b,c)= 1/a+ 1/b+ 1/c= 1

so ∇V = λ∇g ?
so Va = λga
1/6 bc = λ (-1/a2)

so for Vb and Vc, i get
1/6ac = λ(-1/b2)
1/6ab = λ(-1/c2)
Yes, all of this is good.

so since the plane must pass through (1,1,1), i get g(a,b,c) = 1/a + 1/b +1/c = 1

so solving i get a=b=c=3?

so the plane is x/3 + y/3 +z/3 = 1 ?
Well, it would have been better to show how you got that, but, yes, a= b= c= 3 is correct.

but i have a few questions

1) shouldn't V be a function of x,y,z ? and not a,b,c? and so g should also be of x,y,z?
No. (x, y, z) would be a point on the plane. You want to determine which plane which means you want to find a, b, and c.

2) i didn't find the value of λ when i solve the simultaneous equations, is this ok? and even if i find it, what does it symbolize?
Yes, that's fine. In fact often the best way of solving such problems is to immediately eliminate [itex]\lambda[/itex] by dividing your equations. Your condition is that the two gradients are parallel- their individual lengths are not relevant. (lambda is the ratio of those two lengths.)

3) lastly, how do i know that the value a=b=c=3 gives me the minimized volume? why not it be the maximized?

thanks!
Suppose a= A, some fixed large number and that b and c are the same. It is easy to show that [itex]b= c= 2A/(A-1)[/itex and so the volume would be
[itex]\frac{1}{3}\frac{A^3}{(A-1)^2}[/itex]

That goes to infinity as A goes to infinity and so there is NO maximum volume.
 
  • #5
HallsofIvy said:
No. Your condition on the plane x/a+ y/b+ z/c= 1 is that it contains the point (1,1,1).
Your condition is that g(a,b,c)= 1/a+ 1/b+ 1/c= 1 Yes, all of this is good. Well, it would have been better to show how you got that, but, yes, a= b= c= 3 is correct.


No. (x, y, z) would be a point on the plane. You want to determine which plane which means you want to find a, b, and c. Yes, that's fine. In fact often the best way of solving such problems is to immediately eliminate [itex]\lambda[/itex] by dividing your equations. Your condition is that the two gradients are parallel- their individual lengths are not relevant. (lambda is the ratio of those two lengths.) Suppose a= A, some fixed large number and that b and c are the same. It is easy to show that [itex]b= c= 2A/(A-1)[/itex and so the volume would be
[itex]\frac{1}{3}\frac{A^3}{(A-1)^2}[/itex]

That goes to infinity as A goes to infinity and so there is NO maximum volume.

oh i see.

but somehow, i can't see the last point's latex.
the code seems to suggest that b=c=2A/(A-1) if i sub a=A into the g(a,b,c) ? (but why can we let b=c?)

so the volume is 1/3 A3(A-1)2?
but shouldn't it be, V=1/6abc = 1/6 A (2A/(A-1)) (2A/(A-1)) = 2/3 A3(A-1)2?

so you mean that if i can show that V=1/6abc, as long as when a or b or c tends to infinity, if it causes V to tend to infinity, then there is no maximum volume?

also, i was thinking, what if now the axes are not the positive x=0,y=0,z=0 planes making the volume with the plane, but rather, some other planes like x=5. then i just need to find the equation of the volume of the new tetrahedron and do the same right?
 
Last edited:

FAQ: Find plane eqn that minimize Volume

What is the purpose of finding the plane equation that minimizes volume?

The purpose of finding the plane equation that minimizes volume is to determine the smallest possible volume that can be enclosed by a given plane. This can be useful in various applications, such as optimizing space in a design or minimizing material usage in construction.

How is the plane equation that minimizes volume calculated?

The plane equation that minimizes volume can be calculated using mathematical techniques such as calculus and linear algebra. It involves finding the partial derivatives of the volume formula with respect to the plane's parameters and setting them to zero to determine the optimal values.

Can the plane equation that minimizes volume also be used to maximize volume?

No, the plane equation that minimizes volume is specifically for finding the smallest possible volume. To find the largest possible volume, one would need to use a different equation that maximizes volume.

Is the plane equation that minimizes volume unique?

In most cases, yes, the plane equation that minimizes volume is unique. However, there may be certain situations where there are multiple planes that can enclose the same minimum volume. In these cases, any of the planes can be considered the solution.

What are some real-world applications of finding the plane equation that minimizes volume?

Some real-world applications of finding the plane equation that minimizes volume include optimizing space in architecture and design, minimizing material usage in construction, and determining the most efficient way to pack items in a container or storage space.

Back
Top