help

[lilkrazyrae]

I suck at word problems!!!
Can someone help?

A company is constructing an open-top, square-based, rectangular tank that will have a volume of 70ft^3. What dimensions yield the minimum surface area?

 

[Gene]

We could give better help if you told us why you are stuck. We don't know if it is the formulas, if you know the formulas but can't put them together or if you have done that but can't do the differental.
I'll get you started.
The base of the box is a square with sides b. The height is h. The volume is
V = b^2*h = 70
The surface area is the base plus 4 sides.
A = b^2 + 4*b*h
Solve the first equation for h.
Substitute that for the h in the second.
Find dA/db and set it = 0
Solve for b, substitute that in the first to find h
Hint: b is a little more than 5 and h a little less than 3.
If you need more show your work as far as you got.
------------------
Gene

 

[lilkrazyrae]

thanks

Well I could not find the formula for surface area. But thanks for your help!!!

 

[tkhunny]

Re: thanks

1) I suck at word problems!
2) Well I could not find the formula for surface area.

I think you put your finger on part of the problem you were having with this one. Why did you try to "find" the formula. It's a rectangular solid. You probably live in one. Look around you and create the formula for yourself. It's OK. You can use your own correct judgment. You don't need to find the answer in a book. Go stand in your rectangular living room. Imagine measurements marked on the floors and walls. How do they relate to each other? In this case, the ceiling is missing, so don't include that.

Learning to create will help you to abandon the sucking idea. You will still need help from a book, sometimes, maybe even often, but you will gain satisfaction by gaining a measure of independence.