problem using the product rule in calculus

Discussion in 'Community Discussion' started by macman2790, Feb 22, 2007.

  1. macman2790 macrumors 6502a

    macman2790

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    #1
    im having trouble getting the same answer as it says in the back of the book for this one:

    f(x)=(1/(x^2)-3/(x^4))(x + 5x^3)
    im pretty sure that you have to use the product rule on this one.
    the answer in the back of the book is:
    f`(x)= 5 + 14/(x^4)+ 9/(x^2)


    i've also tried multiplying it out and then differentiating it and it gets nowhere near that answer.

    Can someone please explain to me how to get this answer?
    thanks:apple:
     
  2. pianoman181 macrumors regular

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    Jan 6, 2004
    #2
    Don't forget the chain rule!

    If I weren't in bed right now, I'd do it out for you.
     
  3. macman2790 thread starter macrumors 6502a

    macman2790

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    #3
    too bad i cant use it, since it requires the product rule or the other one i stated. i dont know the chain rule, we aren't there yet. sorry
     
  4. macman2790 thread starter macrumors 6502a

    macman2790

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    #4
    by the way the product rule is:
    d/dx [f(x)g(x)] = f(x) d/dx[g(x)] + g(x) d/dx[f(x)]
     
  5. Abstract macrumors Penryn

    Abstract

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    #5
    My advice: While I haven't done this sort of maths for years and years, I think it's best to convert it to this form:

    [(x^-2) - (3x^-4)]*[x + 5x^3]

    This way, you don't need to deal with 1/x type values, as the form above is much easier to deal with when taking derivatives. Can you solve it now?
     
  6. macman2790 thread starter macrumors 6502a

    macman2790

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    #6
    thank you very much
     
  7. siurpeeman macrumors 603

    siurpeeman

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    #7
    are you sure it isn't f'(x) = 5 + 14/(x^2) + 9/(x^4) ?
     
  8. ®îçhå®? macrumors 68000

    ®îçhå®?

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    Mar 7, 2006
    #8
    Unless i am readig it incorrectly, the Chain rule is completely pointless in this case

    Have you ever thought that the book is wrong?? Happens a lot in my book
     
  9. Abstract macrumors Penryn

    Abstract

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    #9

    OK....just did the question myself, and got the same answer as you.

    It actually brought a smile to my face that I could still do it. I had to cheat and use the Product Rule definition given by the OP, but still. :)
     

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