How can I generalize the result for the n=2 case to a larger set of numbers?

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Homework Help Overview

The discussion revolves around generalizing a mathematical result related to ordered sets and ratios. The original poster presents a problem involving elements \(a, b, c, d\) from an ordered set \(K\) and seeks to show that the ratio \(\frac{a+c}{b+d}\) lies between the minimum and maximum of the ratios \(\frac{a}{b}\) and \(\frac{c}{d}\). The goal is to extend this result to a larger set of numbers.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss various approaches to generalizing the result, including the use of inequalities and proof by induction. Some participants explore the implications of rewriting the expression in terms of convex combinations and question how to apply this to the general case.

Discussion Status

The discussion is active, with participants offering different perspectives on how to approach the generalization. Some have provided partial reasoning and insights, while others express challenges in extending their findings to the broader case.

Contextual Notes

There is mention of the constraints of working within an ordered field and the requirement that certain elements are positive. The original poster indicates difficulty in generalizing from the specific case of \(n=2\) to larger sets.

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Homework Statement


a,b,c,d in a ordered set K and b and d>0
Show that [tex]\frac{a+c}{b+d}[/tex] stay between the minimum and max from [tex]\frac {a}{b}[/tex] and [tex]\frac {c}{d}[/tex]. Generalize for [tex]a_1,\hdots,a_n,b_1,\hdots,b_n \in K[/tex] with [tex]b_1\hdots,b_n >0[/tex] so [tex]\frac{a_1+\hdots+a_n}{b_1+\hdots+b_n}[/tex] is between the max and min elements from [tex]\frac{a_1}{b_1},\hdots,\frac{a_n}{b_n}[/tex]

I could do it for the the first case but in a way it's impossible to generalize
any ideas?
tks in advance
 
Last edited:
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to correct, it`s a ordered field
 
if u consider a/b<c/d
u can do a/b - (a+c)/(b+d)}=(ad-bc)/(b(b+d))>0
and c/d - (a+c)/(b+d)}=(bc-ad)/(b(b+d))>0
and done

but them using samething for generalizing i couldn't make it :(
 
Last edited:
Looks like proof by induction would work.
 
Let's redo the n=2 case in a way it will be easier to generalize. Write (a1+a2)/(b1+b2)=(b1/(b1+b2))*(a1/b1)+(b2/(b1+b2))*(a2/b2). Notice that the bi/(b1+b2) terms are positive and sum to 1. (This means (a1+a2)/(b1+b2) is in the 'convex hull' of the bi/ai.) If I replace the ai/bi by their minimum and maximum, what do I conclude?
 

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