Notices
Results 1 to 6 of 6

Thread: Any method for calculating differentiations?

  1. #1 Any method for calculating differentiations? 
    Forum Senior Booms's Avatar
    Join Date
    Sep 2008
    Location
    The perceptual schematic known as earth
    Posts
    361
    about a month ago we were set some homework (so no worries it's overdue now 8) )


    the question was

    if y=ax^3 + bx^2 + cx

    when dy/dx=15 and x=2

    what is a, b and c


    now I still havn't figured it out (forgot to be honest ) BUT I'm not looking for a simple answer to the problem



    the only way I can think of to solve this is using trial and improvement, which is a problem because even assuming a,b,c are not over 5 (which could be wrong) there are still alot of posibilities



    So is there some kind of method or formula to use to work this out? or should I just continue guessing?


    It's not how many questions you ask, but the answers you get - Booms

    This is the Acadamy of Science! we don't need to 'prove' anything!
    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    unfortunately the monks in nepal didn't teach me the secret method of determining unknown constants in differential equations, but i believe you will need 3 conditions to determine your 3 unknwon constants.

    at the moment you only have one condition y' = 15 @ x=2

    are you sure that was the whole problem?


    everything is mathematical.
    Reply With Quote  
     

  4. #3  
    Forum Senior Booms's Avatar
    Join Date
    Sep 2008
    Location
    The perceptual schematic known as earth
    Posts
    361
    sadly yes, although it's possible my teach screwed up again and missed a chunk


    I was toying with the idea of a=-3 b=-2 and c=7.5 which would essentially give the solution y=0 + 0 + (7.5x2) but that I think defeats the purpose
    It's not how many questions you ask, but the answers you get - Booms

    This is the Acadamy of Science! we don't need to 'prove' anything!
    Reply With Quote  
     

  5. #4  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    i think you really have an infinite number of solutions to the problem as it stands.
    someone can correct me if im wrong, but any values which satisfy the equation

    15 = 12a + 4b + c

    are solutions to the problem
    everything is mathematical.
    Reply With Quote  
     

  6. #5 Re: Any method for calculating differentiations? 
    . DrRocket's Avatar
    Join Date
    Aug 2008
    Posts
    5,486
    Quote Originally Posted by Booms
    about a month ago we were set some homework (so no worries it's overdue now 8) )


    the question was

    if y=ax^3 + bx^2 + cx

    when dy/dx=15 and x=2

    what is a, b and c


    now I still havn't figured it out (forgot to be honest ) BUT I'm not looking for a simple answer to the problem



    the only way I can think of to solve this is using trial and improvement, which is a problem because even assuming a,b,c are not over 5 (which could be wrong) there are still alot of posibilities



    So is there some kind of method or formula to use to work this out? or should I just continue guessing?
    If then and if then so there are a lot of ways to make one is a=b=0 and c=15. Another is a=5/4 and b=c=0.
    Reply With Quote  
     

  7. #6  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    didnt i just say that?
    everything is mathematical.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •