Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Logarithmic Differentiation

  1. Apr 21, 2007 #1
    1. The problem statement, all variables and given/known data

    http://img253.imageshack.us/img253/5748/logdiffji4.jpg [Broken]

    3. The attempt at a solution

    I've tried both and i) and ii) however something just doesn't seem right. Can somebody tell me if my method is correct?

    i) http://img253.imageshack.us/img253/8791/img0186anh3.jpg [Broken] ii)http://img329.imageshack.us/img329/627/img0187azr5.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Apr 21, 2007 #2

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You forgot to differentiate.
     
  4. Apr 21, 2007 #3
    I've got a total mindblock :yuck: , can you explain a little further?
     
  5. Apr 21, 2007 #4
    differentiate both right and left hand side...
     
  6. Apr 21, 2007 #5
    Oh yeh! I tried by using chain rule (Let sinx^cosx = u)
    therefore i get :

    (1/y)*(dy/dx) = (1/u)*[d/dx(sinx^cosx)]

    How would I go about diff-ing sinx^cosx?
     
  7. Apr 21, 2007 #6
    consider cosx as u and log(sinx) as v, then diff by the product rule (udv+vdu)
    In diff. v, use the chain rule
     
  8. Apr 21, 2007 #7
    Ah! so much easier!
     
  9. Apr 21, 2007 #8
    Ok, after that tip from f(x) I got what I beleive is the final answer. Can this be simplified any further?

    http://img156.imageshack.us/img156/115/img0192us4.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
  10. Apr 21, 2007 #9

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Your work looks good up until very last step. Try that final step one more time.
     
  11. Apr 21, 2007 #10
    silly me! :rofl:

    2nd time lucky!
    http://img441.imageshack.us/img441/8141/answerem9.jpg [Broken]
     
    Last edited by a moderator: May 2, 2017
  12. Apr 21, 2007 #11

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Much better.
     
  13. Apr 21, 2007 #12
    I'm not too sure how to go about part (ii)
    So far i've converted the square roots to ^1/2 and expanded out to get
    x^(3/2)[(1-x^2)^(1/4)]

    I then let that equal u (chain rule) however the product rule within u seems to get pretty messy/complicated.

    Is this the correct method?
     
  14. Apr 21, 2007 #13

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    If [itex]y=x^{3/2}[(1-x^2)^{1/4}],[/itex] then let [itex]u=x^{3/2} [/itex]and [itex]v=(1-x^2)^{1/4} [/itex] so that y'=uv'+u'v, remembering that to calculate v' you will need to use the chain rule.
     
    Last edited: Apr 21, 2007
  15. Apr 22, 2007 #14
    Ok, so I now know how to diff [itex]y=x^{3/2}[(1-x^2)^{1/4}],[/itex] but shouldn't I take log of both sides before diff-ing? In that case I need to figure out the derivative of ln[[itex]y=x^{3/2}[(1-x^2)^{1/4}],[/itex]]
     
  16. Apr 23, 2007 #15
    Bump, can anybody help?
     
  17. Apr 23, 2007 #16

    HallsofIvy

    User Avatar
    Science Advisor

    You could but you don't have to. Typically "logarithmic differentiation is used when you have a function of x in the exponent.

    Just use the product rule:
    [itex]y'= (x^{3/2})'(1- x^2)^{1/4}+ x^{3/2}[(1-x^2)^{1/4}]'[/itex]
    [itex]= (3/2)x^{1/2}(1- x^2)^{1/4}+ (x^{3/2})(1/4)(1-x^2)^{-3/4}[/itex]

    If you take the logarithm of both sides, you get [itex]ln y= (3/2)ln(x)+ (1/4)ln(1+ x^2)[/itex]. Now [itex](1/y)y'= 3/(2x)+ 2x/(4(1+x^2))[/itex]. That righthand side is much simpler but you still have to multiply by y.
     
  18. Apr 23, 2007 #17

    VietDao29

    User Avatar
    Homework Helper

    Another way to differentiate the second expression is to change it to a single n-root, like this:
    [tex]\sqrt{x ^ 3 \sqrt{1 - x ^ 2}} = \sqrt{\sqrt{x ^ 6 (1 - x ^ 2)}} = \sqrt[4]{x ^ 6 - x ^ 8}[/tex]
    Let u = x6 - x 8. Now, the whole expression becomes [tex]\sqrt[4]{u}[/tex], you can apply the chain rule and finish the problem. Can you go from here? :)
     
  19. Apr 24, 2007 #18
    I think ill go with this method. So using chain rule I get dy/dx = (1/4)((x^6-x^8)^(-3/4))*(6x^5-8x^7)

    Hope you can make that out. So where does the logarithmic part come into it? Do I now take log of both sides?

    I'm quite lost:confused:
     
  20. Apr 24, 2007 #19

    VietDao29

    User Avatar
    Homework Helper

    Nope, you can differentiate it normally as you've just done.

    Or you can tuse logarithmic differentiation as HOI has suggested, i.e, it goes like this:

    [tex]y = \sqrt{x ^ 3 \sqrt{1 - x ^ 2}}[/tex]

    Taking log of both sides yields:

    [tex]\ln y = \ln \left( \sqrt{x ^ 3 \sqrt{1 - x ^ 2}} \right) = \frac{1}{2} \ln \left( x ^ 3 \sqrt{1 - x ^ 2} \right) = \frac{1}{2} \left( \ln (x ^ 3) + \ln( \sqrt{1 - x ^ 2} ) \right)[/tex]

    Now, you can differentiate both sides to find y'.

    There are many ways to approach a differentiation problem, so just choose the one that you like best.
     
  21. Apr 25, 2007 #20
    That's very helpful! Had to navigate through the diff of [tex]\frac{1}{2} \left( \ln (x ^ 3) + \ln( \sqrt{1 - x ^ 2} ) \right)[/tex]. Was a bit tricky but I think I got it...
    http://img253.imageshack.us/img253/6788/asdfry0.jpg [Broken]

    Can you please confirm?
     
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook