• Support PF! Buy your school textbooks, materials and every day products Here!

Level surfaces of a function

  • Thread starter kasse
  • Start date
366
0
1. Homework Statement

Describe the level surfaces of f(x,y,z) = z + sqrt(x^2 + y^2)

3. The Attempt at a Solution

First of all, what is actually a level surface? Just a normal surface in space?

I followed an example I found on the internet, and this is my attempt at a solution:

First replace f(x,y,z) with a constant

k = z + sqrt(x^2 + y^2)

Then square (k is now another constant)

k = z^2 + x^2 + y^2

This is an ellipsoid, so the level surfaces are ellipsoids centered at the origin.

Is this the right solution? If so, is it possible to say more about the ellipsoids?
 

Answers and Replies

matt grime
Science Advisor
Homework Helper
9,394
3
The definition of a level surface of function of f(x,y,z) is the solutions to f(x,y,z)=k for a constant k.

Now, please don't tell me that you think (a+b)^2=a^2+b^2, as you wrote above....
 
366
0
OK, so

k = z^2 + 2sqrt(x^2+y^2) + x^2 +y^2

then.

Is this one easy to recognize as a 3D-figure?
 
Last edited:
Dick
Science Advisor
Homework Helper
26,258
618
[
k = z + sqrt(x^2 + y^2)
Get the radical all by itself on one side of the equation before you square. It will be MUCH easier to recognize.
 
366
0
Get the radical all by itself on one side of the equation before you square. It will be MUCH easier to recognize.
Then i get x^2 + y^2 - z^2 = k^2 - 2kz

Still doesn't resemble anything I'm familiar with.
 
Dick
Science Advisor
Homework Helper
26,258
618
Leave it as x^2+y^2=(k-z)^2. x^2+y^2 is often called r^2, right? So write this as r=|z-k|. Can you describe it now?
 
366
0
A sphere, isn't is?
 
Dick
Science Advisor
Homework Helper
26,258
618
A sphere, isn't is?
Nope. Look, r is the radius in the xy plane, right? If r=0 then z=k. What is z for r=1? Note k-z must be positive. Why?
 

Related Threads for: Level surfaces of a function

Replies
2
Views
4K
Replies
1
Views
3K
Replies
3
Views
2K
Replies
7
Views
567
  • Last Post
Replies
7
Views
4K
  • Last Post
Replies
4
Views
725
  • Last Post
Replies
16
Views
9K
Replies
1
Views
2K
Top