# Optimization: Open Box help

1. Nov 22, 2009

### Slimsta

1. The problem statement, all variables and given/known data
http://img7.imageshack.us/img7/1826/43544187.jpg [Broken]

2. Relevant equations

3. The attempt at a solution
whats wrong with my answers? everything looks right to me... :S

Last edited by a moderator: May 4, 2017
2. Nov 23, 2009

### HallsofIvy

The only thing wrong that I see is that you haven't answered the question. You said that the volume will be minimized. You have given x, but you haven't found y and have not said what the dimensions are.

3. Nov 24, 2009

### Slimsta

y=2500/17.12

the thing is, i only have to choose the right answer from those drop down menu boxes..
so i must of chosen something wrong.. but what? i dont see any mistakes

4. Nov 24, 2009

### Slimsta

okay my derivatives look fine.. when f'(x)=0 x=17.1
f''(x) > 0 for x>0.. thats right because if plugging in a negative number i will get f''(x) = -..

so whats wrong?

5. Nov 24, 2009

### Slimsta

6. Nov 25, 2009

### Slimsta

why are u guys ignoring this post? is it something that i said?
for the last part where it says it will be relative min, when x=___
would it be -21.5446 ?
i got it by getting the second derivative equal to 0

7. Nov 25, 2009

### Staff: Mentor

One of your entry boxes says "This implies that the surface area is given in S only..."
Except for this, everything else it looks fine.

Here's a tip you might consider. Many or most of the problems you have posted have oddball numbers such as a volume of V = 2500.1055 cm^3.
I did all of my calculations using V, and replaced V only in the very last step. This saved my from writing 2500.1055 a bunch of times.

For example, A = x^2 + 4V/x. It's easy to get dA/dx = 2x -4V/x^2. Rewriting this as dA/dx = 2x -4Vx-2, it's easy to get the second derivative and verify that it's positive for all x > 0.

8. Nov 25, 2009

### Slimsta

i did it this way.. i substituted V only at the very end and i got the same answers..
and about the "This implies that the surface area is given in S only..." yeah i didnt read it carefully but still. now i got it tnx!

Last edited: Nov 25, 2009