# Thread: I really need help with this! :(

1. ## I really need help with this! :(

I'm stuck on these questions and need someone to show me how to do them. It's not that i was too lazy to do my homework, i've been off school for ages due to an illness and missed a lot so i don't know how to do these kind of questions yet. Hopefully if i see a demonstration i will be able to get the hang of it.

Q.1: An open box with a square base has the volume 256cm^3.
a) Taking x as the length of the base, show that the surface area of the box is x^2+1024/x

[[ This part i figured out ]]

b) Find the dimensions of the box that will minimise the surface area and calculate this area.

[[ The answer is 8 cm by 8cm by 4 cm, area= 192 cm .... how?]]

Q.2: The cost in millions, of pumping water at a hydroelectric station is related to the radius of the pipe carrying the water.
For a pipe the radius of r metres the cost is given by C = r + 9/r + 7
Find the radius which minimises the cost and calculate this cost.

[[ Answer is r = 3 metres, cost = £13 million ]]

Thank you (:

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EDIT:

Never mind!! After i spent ages waiting for someone to answer i ended up figuring it out myself!!

2. Originally Posted by VintageSailor
I'm stuck on these questions and need someone to show me how to do them. It's not that i was too lazy to do my homework, i've been off school for ages due to an illness and missed a lot so i don't know how to do these kind of questions yet. Hopefully if i see a demonstration i will be able to get the hang of it.

Q.1: An open box with a square base has the volume 256cm^3.
a) Taking x as the length of the base, show that the surface area of the box is x^2+1024/x

[[ This part i figured out ]]

b) Find the dimensions of the box that will minimise the surface area and calculate this area.

[[ The answer is 8 cm by 8cm by 4 cm, area= 192 cm .... how?]]

Q.2: The cost in millions, of pumping water at a hydroelectric station is related to the radius of the pipe carrying the water.
For a pipe the radius of r metres the cost is given by C = r + 9/r + 7
Find the radius which minimises the cost and calculate this cost.

[[ Answer is r = 3 metres, cost = £13 million ]]

Thank you (:

------------------------------------------------------------------
EDIT:

Never mind!! After i spent ages waiting for someone to answer i ended up figuring it out myself!! Thanks for your help!! This forum is ****!
An hour and a half is technically not ages... There just might not have been anyone here at the time...

3. ## Lol

Err.. sorry about my mini rage there. I'd previously posted the question 4 times on yahoo answers and no one replied at all even 3 hours later so i was pretty p****d off as you can imagine. I got the answer myself eventually, but it took ages, because i haven't learned how to do these questions at all. I thought if i joined a forum dedicated to maths, people would eagerly help me out....

4. Originally Posted by VintageSailor
Err.. sorry about my mini rage there. I'd previously posted the question 4 times on yahoo answers and no one replied at all even 3 hours later so i was pretty p****d off as you can imagine. I got the answer myself eventually, but it took ages, because i haven't learned how to do these questions at all. I thought if i joined a forum dedicated to maths, people would eagerly help me out....
You're upset because no-one at yahoo answered after 3 hours? No, actually I can't imagine. Perhaps if you'd been guaranteed to get an answer within a couple of hours and had paid good money up front on that expectation, then I might understand.

The fact is that you're getting upset because no-one has has given you help (for free) within just a couple of hours of you posting your question. This sort of impatient attitude and sense of entitlement is not acceptable in any community, whether it be yahoo or MHF.