1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Chain rule =/

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data
    Hi all , again i got stuck onto this iffy bit , question is:
    find dy/dx of the curve and hence its coordinates of the statinonary point.
    y= 4 - 2/(2x-1)^2 -x
    2. Relevant equations
    du/dx x dy/du

    3. The attempt at a solution
    well this is how i started off:
    let u = (2x-1) , y= 4-2(u)^-2 - x
    du/dx= 2;dy/du:4u^-3-1

    [8u^(-3) -2]
    to find stationary points i will let dy/dx= 0 and solve the equation ?am i on the right track so far?
  2. jcsd
  3. Dec 12, 2009 #2
    oo i think i have made a mistake.. i should have multiplied gradient u (2) with the first term for the gradient of dy/du ?
    EDIT:sorry for being such a spammer,i managed to solve this question but grr.. i am stuck on the next one.
    question is:
    curve y = (3x-2)^3 - 5x^2
    find dy/dx and stationary points.

    My attempt :
    dy/dx= 9u^2 - 10x
    we know u= (3x-2), so equation becomes
    9(3x-2)^2- 10x
    81x^2 -108x +26= 0
    divide by 9
    9x^2 -12x -2.8 ( am i over-complicating it?)
    Last edited: Dec 12, 2009
  4. Dec 13, 2009 #3


    User Avatar
    Science Advisor

    Check your arithmetic! 9(9x^2- 12x+ 4)- 10x= 81x^2- 108x+ 36- 10x.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook