Find the formula for an affine function

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Homework Help Overview

The discussion revolves around understanding affine functions, specifically in the context of a mapping from \(\mathbb{R}^{2}\) to \(\mathbb{R}\). Participants are exploring the properties and implications of such functions, particularly in relation to geometric interpretations involving parallelograms.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants express confusion regarding the expression and properties of affine functions. There is an attempt to clarify the form of an affine function and its relation to vectors in an affine space. Questions are raised about the relationship between the function values at points corresponding to the vertices of a parallelogram.

Discussion Status

The discussion is ongoing, with participants providing insights into the nature of affine functions and exploring geometric interpretations. Some guidance has been offered regarding the relationship between function values and the geometry of parallelograms, but no consensus has been reached on the correctness of specific interpretations.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information they can use or reference. There is an emphasis on understanding the definitions and properties of affine functions without providing direct solutions.

Aleoa
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Homework Statement



Schermata 2018-04-12 11:02:55.png

The Attempt at a Solution


I don't understand what an affine function \mathbb{R}^{2}\rightarrow \mathbb{R} expresses... I don't know if it's a coordinate function.

And i also don't understand (i'm sorry) why for any parallelogram, u(P) + u(R) = u(Q) + u(S)
 

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Aleoa said:
I don't understand what an affine function \mathbb{R}^{2}\rightarrow \mathbb{R} expresses...
It's a function of the form ##\pmatrix{x\\y}\mapsto ax+by+c## for some real constants ##a,b,c##.
And i also don't understand (i'm sorry) why for any parallelogram, u(P) + u(R) = u(Q) + u(S)
This may be easier to understand if you deduct (u(R)+u(Q)) from both sides and then think of it in terms of vectors that are edges of the parallelogram.

An affine space is one in which the difference of any two elements is a vector, so it's usually easier to work with things in an affine space when they are expressed in terms of the difference of two elements. Performing the operation described in the previous paragraph does just that.
 
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andrewkirk said:
It's a function of the form ##\pmatrix{x\\y}\mapsto ax+by+c## for some real constants ##a,b,c##.

This may be easier to understand if you deduct (u(R)+u(Q)) from both sides and then think of it in terms of vectors that are edges of the parallelogram.

An affine space is one in which the difference of any two elements is a vector, so it's usually easier to work with things in an affine space when they are expressed in terms of the difference of two elements. Performing the operation described in the previous paragraph does just that.

Thanks so much. The formula is F=3x-y+2.

And, the lines u = constant in R^2 are all the lines perpendicular to the vector [1,3]. Is it correct ?
 
Aleoa said:
the lines u = constant in R^2 are all the lines perpendicular to the vector [1,3]. Is it correct ?
Perpendicular? or Parallel? Check which it is by substituting in a couple of values on the line y=3x, then a couple on a line perpendicular to that, and see for which pair the function gives the same value.
 
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