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: Delta-Epsilon Multivariable

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data
    Let f(x,y)=2x+3y.
    Let [tex]\epsilon[/tex] be any positive number. Show that there is a disk with center (1,1) such that whenever P is in that disk, [tex]|f(P)-5|< \epsilon[/tex]. (Give [tex]\delta[/tex] as a function of [tex]\epsilon[/tex].)

    2. Relevant equations

    3. The attempt at a solution
    Um, I tried to rewrite stuff in a form that's needed, but I can't really get anything. My trouble with these problems is setting everything up and then rearranging it cleverly to get what we need.
  2. jcsd
  3. Oct 16, 2008 #2


    User Avatar
    Homework Helper

    |f(x,y) - 5| = |(2x-2)+(3y-3)| \le 2|x-1| + 3|y-3|

    so now ...
  4. Oct 16, 2008 #3
    So that is epsilon? How would we find the radius of the disk then?
  5. Oct 17, 2008 #4


    User Avatar
    Science Advisor

    The form statdad gave gives you almost immediately the dimensions of a rectangle that will work. Can you find the radius of a disk that will be contained in that rectangle?

    By the way, you say "the" disk. You are only asked to find the radius of "a" disk. There are an infinite number that will work.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook