Proving that a line is straight

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Discussion Overview

The discussion revolves around methods to prove that three points are collinear, focusing on theorems and properties that can be applied in various mathematical contexts. Participants explore different approaches to establish whether the points lie on the same line.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant asks about theorems or properties that can be used to prove that three points are collinear.
  • Another participant suggests calculating the slopes between pairs of points and checking if they are equal to determine collinearity.
  • A different participant challenges the slope method by noting that parallel lines have the same slope but do not intersect, proposing instead to find the equation of a line and check if the other points satisfy it.
  • Additional properties mentioned include the area of the triangle formed by the three points being zero, the vector cross product of vectors joining the points being zero, and a specific linear combination of position vectors.
  • One participant reiterates the point about parallel lines, emphasizing that in the context of the previous method, the lines must intersect since they share a common point.

Areas of Agreement / Disagreement

Participants express differing views on the validity of using slopes to determine collinearity, with some supporting the method while others raise concerns about its limitations. The discussion remains unresolved regarding the best approach to prove that three points are on the same line.

Contextual Notes

The discussion includes various mathematical properties and methods, but there are unresolved assumptions regarding the definitions of collinearity and the applicability of certain methods in different scenarios.

daniel_i_l
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In various riddles and math questions you're asked to prove that 3 points are on the same line. what theorms can be used to prove this sort of thing. in other words, what properties of the points can be used to prove that they're all on a line?
Thanks.
 
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Choose two of them find the slope of those two points, pick two others(one of the originals and the other one) and find the slope between those two, if the slopes are the same they lie on the same line.
 
d_leet said:
Choose two of them find the slope of those two points, pick two others(one of the originals and the other one) and find the slope between those two, if the slopes are the same they lie on the same line.

Ah... No.

Parallel lines have the same slope, but never intersect.

Find the equation of a line between 2 of the points.

Do the other points satisfy the equation? If so, they are on the line.
 
Some other properties that might be useful:

Area of the triangle defined by the 3 points = 0

Vector cross product of two vectors joining the points = 0

If the points have position vectors a b and c in order along a line, then b = ka + (1-k)c for some constant k with 0 < k < 1.
 
Integral said:
Parallel lines have the same slope, but never intersect.

True, but in d_leet's method the two lines always intersect because they have a common point.
 

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