Results 1 to 4 of 4

Math Help - I really need help with this! :(

  1. #1
    Banned
    Joined
    Nov 2009
    Posts
    2

    Exclamation 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 (:

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

    Never mind!! After i spent ages waiting for someone to answer i ended up figuring it out myself!!
    Last edited by VintageSailor; November 2nd 2009 at 08:56 AM. Reason: err sorry for being rude
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Prove It's Avatar
    Joined
    Aug 2008
    Posts
    10,969
    Thanks
    1009
    Quote Originally Posted by VintageSailor View Post
    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...
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Banned
    Joined
    Nov 2009
    Posts
    2

    Smile 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....
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by VintageSailor View Post
    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.
    Follow Math Help Forum on Facebook and Google+

Search Tags


/mathhelpforum @mathhelpforum