Results 1 to 7 of 7
Like Tree2Thanks
  • 1 Post By ibdutt
  • 1 Post By Soroban

Math Help - Finding derivative with log

  1. #1
    Junior Member
    Joined
    Apr 2013
    From
    Melbourne
    Posts
    29
    Thanks
    1

    Finding derivative with log

    Can't seem to get this question.

    find dy/dx of y=-log(base,e)(cosx)

    i get the expression to (-cosx/x)+log(base,e)+sinx. But I don't know how to take it any further.

    the answer is tanx

    any help appreciated, thx
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    Joined
    Jul 2012
    From
    INDIA
    Posts
    834
    Thanks
    209

    Re: Finding derivative with log

    You are probably going wrong
    we have y = - log cosx = log ( cosx)^-1= log sec x
    dy/dx = (1/sec x )( sec x tan x ) = tan x
    remember differential of log ( f(x)) = (1/f(x) ) f'(x)
    Thanks from JellyOnion
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member

    Joined
    May 2006
    From
    Lexington, MA (USA)
    Posts
    11,908
    Thanks
    766

    Re: Finding derivative with log

    Hello, JellyOnion!

    Another way . . .


    \text{Differentiate: }\:y \:=\:-\ln(\cos x)

    \frac{dy}{dx} \;\;=\;\;-\frac{1}{\cos x}(-\sin x) \;\;=\;\;\frac{\sin x}{\cos x} \;\;=\;\;\tan x
    Thanks from JellyOnion
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Jul 2013
    From
    Leeds, uk
    Posts
    2

    Re: Finding derivative with log

    What you need is the product rule. So d/dx (f(g(x))) is f'(g) x g'(x). log'(f) is 1/f, d/dx(cos) is -sin,
    so you have -sin/-cos , which is tan.

    For more info, please visit:-

    Maths Discovery
    Follow Math Help Forum on Facebook and Google+

  5. #5
    MHF Contributor

    Joined
    Apr 2005
    Posts
    16,413
    Thanks
    1852

    Re: Finding derivative with log

    That's NOT the "product rule", that is the "chain rule".
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Forum Admin topsquark's Avatar
    Joined
    Jan 2006
    From
    Wellsville, NY
    Posts
    10,211
    Thanks
    419
    Awards
    1

    Re: Finding derivative with log

    Quote Originally Posted by bobm2013 View Post
    What you need is the product rule. So d/dx (f(g(x))) is f'(g) x g'(x). log'(f) is 1/f, d/dx(cos) is -sin,
    so you have -sin/-cos , which is tan.

    For more info, please visit:-
    I find this comment rather ironic since he's advertising a paid tutoring service.

    -Dan
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Junior Member
    Joined
    Apr 2013
    From
    Melbourne
    Posts
    29
    Thanks
    1

    Re: Finding derivative with log

    Lol, thx everyone. You guys helped heaps for my test today. Cheers
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. finding derivative
    Posted in the Calculus Forum
    Replies: 3
    Last Post: June 4th 2012, 07:15 AM
  2. Finding a derivative
    Posted in the Calculus Forum
    Replies: 5
    Last Post: January 4th 2010, 03:13 AM
  3. Finding a derivative
    Posted in the Calculus Forum
    Replies: 1
    Last Post: October 9th 2009, 11:32 PM
  4. Need help finding the derivative.
    Posted in the Calculus Forum
    Replies: 3
    Last Post: June 10th 2009, 12:44 PM
  5. Finding this derivative
    Posted in the Calculus Forum
    Replies: 2
    Last Post: December 3rd 2008, 12:34 PM

Search Tags


/mathhelpforum @mathhelpforum