Results 1 to 9 of 9

Math Help - Help with Deritvitives dealing with limits

  1. #1
    Junior Member
    Joined
    Oct 2007
    Posts
    31

    Help with Deritvitives dealing with limits

    Got caught up here... would greatly appreciate some help in working through these...

    An object moves along so that it's location is -5x + 2x . Find velocity at following points: 1, 3, 5.
    This is using derivatives and finding the velocities at the points. I've used what i know to find the slope of the tangent and secant line... unless I did my math wrong thats not how to find the answer. Any advise please?

    Also, if someone can show me a method of solving a derivitive with a fraction, that'd be great.

     Ex. f(x) = \frac {1}{x + 2}
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member 11rdc11's Avatar
    Joined
    Jul 2007
    From
    New Orleans
    Posts
    894
    I'm not sure what you asking but just take the derivative of your position function to get your velocity function.

    From there just plug in the points given to you into your velocity function to find it velocity.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Oct 2007
    Posts
    31
    I did exactly as you said, and the correct answer has not came up for me.

    And I could really use help on solving with a fraction if possible.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member 11rdc11's Avatar
    Joined
    Jul 2007
    From
    New Orleans
    Posts
    894
    f(x) = \frac {1}{x + 2} = \frac{u}{v}

    using quotient rule

    \frac{(du)(v) - (u)(dv)}{v^2}


    -\frac{1}{(x + 2)^2}
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Super Member 11rdc11's Avatar
    Joined
    Jul 2007
    From
    New Orleans
    Posts
    894
    For your position function

    f(x) = - 5x + 2x

    f(x) = -3x

    now take the derivative

    f'(x) = -3

    so the velocity is -3 at any time
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Junior Member
    Joined
    Oct 2007
    Posts
    31
    Sorry, bad typo... i mean [tex] -5x^2 +2x [tex]
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Math Engineering Student
    Krizalid's Avatar
    Joined
    Mar 2007
    From
    Santiago, Chile
    Posts
    3,654
    Thanks
    11
    Having f(x)=\frac1{x+2}, it's a worth of try on writtin' it as f(x)=(x+2)^{-1}, now this is a matter of power rule + chain rule.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Super Member 11rdc11's Avatar
    Joined
    Jul 2007
    From
    New Orleans
    Posts
    894
    Thanks Krizalid but I remember learning quotient rule 1st before learning chain rule so thought he may not know how to apply chain rule just yet.

    f(x) = -5x^2 +2x

    f'(x) = -10x + 2

    Now plug in your points {1,3,5}

    f'(1) = -8
    f'(3) = -28
    f'(5) = -48

    Just wondering you said your doing derivatives using limits so do you want to see the the derivative found like this?



    \lim_{h \to 0} \frac{[-5(x + h)^2 + 2(x + h)] -[ -5x^2 +2x ]}{h}
    Last edited by 11rdc11; September 8th 2008 at 04:53 PM.
    Follow Math Help Forum on Facebook and Google+

  9. #9
    Junior Member
    Joined
    Oct 2007
    Posts
    31
    That did the trick... although that's what I seemed to be doing the entire time. However... it appears i got the (-) mixed up while distributing.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. HW help....dealing with e
    Posted in the Calculus Forum
    Replies: 2
    Last Post: March 29th 2010, 11:35 AM
  2. Dealing with
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: November 3rd 2009, 04:35 AM
  3. Dealing with Multivariate Limits?
    Posted in the Calculus Forum
    Replies: 1
    Last Post: April 14th 2009, 12:44 AM
  4. Dealing with infinite limits.
    Posted in the Calculus Forum
    Replies: 1
    Last Post: October 28th 2008, 12:22 AM
  5. dealing with -i
    Posted in the Algebra Forum
    Replies: 4
    Last Post: May 15th 2008, 01:06 PM

Search Tags


/mathhelpforum @mathhelpforum