How to Find the Radius of a Disk for Delta-Epsilon Proof?

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Homework Help Overview

The problem involves a function f(x,y) = 2x + 3y and requires demonstrating the existence of a disk centered at (1,1) such that for any point P within that disk, the condition |f(P) - 5| < ε holds true. The task includes expressing δ as a function of ε.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss rewriting the expression |f(x,y) - 5| and attempt to relate it to ε. There is a focus on setting up the problem and rearranging terms effectively. Questions arise about how to determine the radius of the disk and the implications of the rectangle formed by the inequalities.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some have provided partial expressions and are questioning how to derive the radius of the disk from the established inequalities. There is recognition that multiple disks could satisfy the condition, emphasizing the exploratory nature of the task.

Contextual Notes

Participants note the challenge in setting up the problem correctly and the absence of specific equations or methods provided in the homework statement. The focus remains on understanding the relationship between the disk's radius and the established inequalities.

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Homework Statement


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].)


Homework Equations


None.


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.
 
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[tex] |f(x,y) - 5| = |(2x-2)+(3y-3)| \le 2|x-1| + 3|y-3|[/tex]

so now ...
 
So that is epsilon? How would we find the radius of the disk then?
 
statdad said:
[tex] |f(x,y) - 5| = |(2x-2)+(3y-3)| \le 2|x-1| + 3|y-3|[/tex]

so now ...

dtl42 said:
So that is epsilon? How would we find the radius of the disk then?

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.
 

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