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: Maxima and minima of functions of two variables

  1. Nov 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Locate all relative maxima, relative minima ,and saddle points if any.




    ycosx=0 sinx=0
    y=0 x=0,∏,2∏... up until infinity
    Critical points at (0,0),(∏,0),(2∏,0)....


    fyy(x,y)= cosx

    fxx*fyy-f(x,y)2→0-0=0 ∴ no relative extrema,

    however the book says that fxx*fyy-f(x,y)2=-1 which means saddle point.

    I dont understand if y is = to zero.. the function ysinx will always be zero..

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 5, 2012 #2
  4. Nov 5, 2012 #3
    Sorry that was a typo, but still doesn't help regardless.
  5. Nov 5, 2012 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Isn't fy(x,y) = sin(x) ?

    Then what is fyy(x,y) ? Isn't it zero?

    Also, What is fxy(x,y) ?

    You need fxx*fyy-(fxy)2 , not fxx*fyy-f(x,y)2
    Last edited: Nov 5, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook