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!

More Partial Derv.

  1. Oct 30, 2003 #1
    I am completly stuck on these. I am supposed to find the directional derv. of 1+2x(y)^(1-2) with point (3,4) and vector (4,-3)

    I understand the formula to find this. You have to find the partial derv. of x and y then plug in (3,4) to get a value. Then take the partial derv. of x and multiply it by a, and partial derv. of y multiplied by b.

    fx(3,4)a+fy(3,4)b

    fx=partial derv of x
    fy = partial derv. of y

    the answer in the back of the book is 23/10, I get 23/3. So it must be in my partials? any help is very much apperciated.
     
  2. jcsd
  3. Oct 30, 2003 #2

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Don't forget that your direction vector is supposed to be a unit vector.

    (4, -3) isn't quite a unit vector, so you have to scale it so it is a unit vector before you use it in the problem.


    You might want to check the rest of your work again too.
     
    Last edited: Oct 30, 2003
  4. Oct 30, 2003 #3
    What can I do to the vector (4,-3) to make it a unit vector? Multiply it till the legnth is 1?
     
  5. Oct 30, 2003 #4
    Basically.
    You want sqrt(x2+y2)=1 where x and y are the i and j components of the vector.
    So
    x2+y2=1
    Presently you have
    x2+y2=25
    If you divide each side by 25 then
    sqrt((x/5)2+(y/5)2)=1
    So if the vector is (4/5,-3/5) then you have a unit vector.
     
  6. Oct 30, 2003 #5
    Thank you so much. If I could buy you a beer and cigar I would. That is all I needed, know it all makes sense. I can not tell you how much I am greatfull. I have that feeling of solving that ever so long math problem, it's great.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: More Partial Derv.
  1. Partial Fractions (Replies: 10)

  2. Partial derivative (Replies: 8)

  3. Partial Derivatives (Replies: 5)

  4. Partial Fractions (Replies: 7)

Loading...