A Match Box... differentiation question

1. Jul 12, 2015

lionely

1. The problem statement, all variables and given/known data
a match box consists of an outer cover, open at both ends, into which slides a rectangular box without a top. The length of the box is one and a half times its breadth, the thickness of the material is negligible, and the volume of the box is 25cm^3 . If the breadth of the box is x cm, find , in terms of x, the area of the material used. Hence show that, if the least area of material is to be used to make the box , the length should be 3.7cm approximately.

2. Relevant equations

3. The attempt at a solution
So I have this so far:

25 = 3/2 x^2 h [Volume and h is the height]

For the surface area now, I figured I would find the surface area of the outer box(box1) and the inner box(box2)
and then add them together.

so I have this :

box1 = ((3x^2)/2 + xh) (2) = 3x^2 + 2xh

box 2 = (3x^2)/2 + xh(2) + (3xh/2)(2) = (3x^2)/2 + 5xh

box1 + box2 = (9x^2)/2 + 7xh [ h = (50)/(3x^2) , from the volume ]

Area(A) = (9x^2)/2 + 350/(3x)

dA/dx = 9x - (350)/(3x^2)

for max/min dA/dx = 0

9x = 350/(3x^2)

x^3 = 350/27
x = 2.45cm (which is incorrect)

Where did I go wrong?

Last edited: Jul 12, 2015
2. Jul 12, 2015

Dr. Courtney

Take more care.

Define your variables explicitly.

Recognize your objective function.

Recognize your constraint function.

Use your constraint to reduce your objective function to one variable.

Differentiate.

Set equal to zero.

Solve.

Test each root to be max or min.

3. Jul 12, 2015

lionely

I basically did that, but is my reasoning about the areas correct?

4. Jul 12, 2015

SammyS

Staff Emeritus
For one thing:
You added 2xh + 5xh and got 6xh.

Also, be careful to use parentheses where needed.

5. Jul 12, 2015

Dr. Courtney

Student: "Teacher, teacher, find my mistake."

Teacher: "Learn to take care, use a better problem solving method, and find your own mistake."

Doing it your way leads to dependence and less competence.

Doing it my way brings mastery and independence.

Which do you want in life?

6. Jul 12, 2015

lionely

Okay I will try again before I come back , thank you

7. Jul 12, 2015

lionely

Did it over about 3 times. I believe the surface area expression is correct. So I just don't understand why I can't get 3.7cm.

8. Jul 12, 2015

SammyS

Staff Emeritus
You also have a mistake in the area of the cover (box 1).

9. Jul 12, 2015

lionely

I don't see the mistake, aren't the areas of the top and bottom of the cover (3x/2)(x)(2) and the area of the sides are (x*h)(2)?

10. Jul 12, 2015

SammyS

Staff Emeritus
No.

The side is the the longest dimension times the height.

11. Jul 12, 2015

lionely

So that means the expression for the area = (9x^2)/2 + (400)/(3x)

dA/dx = 9x - (400)/(3x^2)

for max/min dA/dx = 0

9x = 400/(3x^2)
x^3 = 400/27
x = 2.46cm .

I still can't get 3.7cm

12. Jul 12, 2015

SammyS

Staff Emeritus
Look at the problem statement.

Have you found why you're not getting 3.7cm ?

( What is 2/3 of 3.7 ? )

Last edited: Jul 12, 2015
13. Jul 12, 2015

lionely

2/3 of 3.7 is the answer I have... but why would my answer be 2/3 of the length?

14. Jul 12, 2015

SammyS

Staff Emeritus
You're comparing your answer to the wrong dimension.

15. Jul 12, 2015

lionely

Ah I see now the problem I had was with the english. When I saw length I thinking about a piece of cardboard that long, but when they said length it meant... the length of the box. Thank you, also this might be quite stupid but I don't fully see how the area for the sides of the outerbox is the longest dimension times the height. Why isn't the breadth*height. I thought the outerbox didn't have ends so I couldn't do the former.

16. Jul 13, 2015

SammyS

Staff Emeritus
It's just a matter of which sides are "missing". The two missing sides are those with smallest area. That's common for this type of matchbox.

Last edited: Jul 13, 2015
17. Jul 13, 2015

BvU

Nice thread. "the problem I had was with the english" seems a bit strange to me, since your english appears excellent to me. To me it looks as if the step from what you read to what you imagined went awry. Would it have helped if you had made a sketch of the kind of box you had in mind ?

I like dr C's advice -- as a generic motto. Perhaps it should include "make a drawing".

Nowadays no one in his right mind smokes anymore and matchboxes are almost extinct. Goes to show problem authors should be more aware of the cultural context of their poor victims...

18. Jul 13, 2015

Staff: Mentor

I couldn't agree more. On almost every applied calculus problem it's a good idea to make a sketch. Beginning students seem to be resistant to this idea. Granted, it takes more effort at first, but it's a false economy to save time by not drawing a sketch, and then waste much more time going down paths that lead to a wrong answer.

Also, I believe that having a sketch to look at utilizes the half of the brain that isn't used in symbolic calculations.