Proving that a line is straight (1 Viewer)

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

daniel_i_l

Gold Member
865
0
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.
 
1,074
1
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.
 

Integral

Staff Emeritus
Science Advisor
Gold Member
7,161
54
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.
 

AlephZero

Science Advisor
Homework Helper
6,920
290
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.
 

AlephZero

Science Advisor
Homework Helper
6,920
290
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.
 

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top