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

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SUMMARY

Three points P, Q, and R are collinear if and only if the cross product of vectors PQ and PR equals zero, expressed mathematically as vec PQ x vec PR = 0. This condition indicates that the angle between the two vectors is either 0 or 180 degrees, confirming their alignment. The relationship can be derived from the formula for the cross product, which incorporates the sine of the angle between the vectors. Understanding this concept is crucial for solving problems related to vector geometry.

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hachi_roku
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Homework Statement


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


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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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axb=|a||b|sinθ

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

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