Showing that three points are collinear if their cross product is zero

In summary, the cross product of three points is a mathematical operation that results in a vector perpendicular to the original vectors formed by the points. It can be used to show collinearity by checking if the resulting vector is parallel to the third point. If the cross product is zero, it means the points are coplanar and collinear. It is not possible for the cross product to be zero if the points are not collinear. While there are other methods for proving collinearity, using the cross product is a reliable and efficient approach.
  • #1
hachi_roku
61
0

Homework Statement


show that three points P,Q, and R are collinear if and only if vec PQ x vec PR = 0


Homework Equations





The Attempt at a Solution


i understand this (i can see this visually in my head) but i just don't know how to say it mathematically...i know that the angle between them must be 0 or 180, but how do i show this?
 
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  • #2
axb=|a||b|sinθ

so what is the angle between PQ and PR?
 
  • #3
thanks haha =)
 

1. What is the cross product of three points?

The cross product of three points is a mathematical operation that results in a vector that is perpendicular to both of the original vectors formed by the points. It is denoted by the symbol "x" and is calculated using a specific formula.

2. How can I show that three points are collinear using their cross product?

To show that three points are collinear using their cross product, you can calculate the cross product of any two of the points and then check if the resulting vector is parallel to the third point. If it is parallel, then the points are collinear.

3. What does it mean if the cross product of three points is zero?

If the cross product of three points is zero, it means that the three points are coplanar, or lying on the same plane. This also indicates that the points are collinear, since a plane can only contain collinear points.

4. Can the cross product of three points be zero if the points are not collinear?

No, the cross product of three points can only be zero if the points are collinear. This is because the cross product is only zero when the two vectors being multiplied are parallel to each other.

5. Are there any other ways to prove that three points are collinear besides using their cross product?

Yes, there are other ways to prove that three points are collinear, such as using the slope formula to show that the slopes between the points are equal, or by showing that the distance between each point is equal. However, using the cross product is a reliable and efficient method for proving collinearity.

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