Vector Proof: x x v = u x v = x x u

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Homework Help Overview

The problem involves proving a vector identity related to the cross product, specifically showing that if \( x + v = u \), then \( x \times v = u \times v = x \times u \). The subject area is vector algebra, particularly focusing on properties of the cross product.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss various approaches to the problem, including the use of vector properties and the associative rule. Some express uncertainty about how to start, while others suggest substituting known values and simplifying expressions.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations and methods. Some have provided hints and suggestions for approaching the proof, while others are still grappling with the concepts involved.

Contextual Notes

There is mention of the need to consider both magnitude and direction in the context of the cross product. Additionally, participants are unsure about the constraints on methods allowed for the proof, such as whether to use coordinates or properties of the cross product.

Jbreezy
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Homework Statement



If x + v = u
Prove x x v = u x v = x x u

The Attempt at a Solution



I don't even know where to start with this. I thought that magnitude of the resultant vector would have to be equal. So I started messing with each to see if I could find a pattern.

x x v = | x|| v| sin θ

This is a crap approach I can't find anything. Please give me a hint not the answer. I just can't seem to draw the information in my text together to give myself enough to show this. Thank you.
 
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Just use your definition of u in the equations, and simplify with known properties of the cross product.
It does not help to consider the magnitude of the vectors, as their "direction" has to fit, too.
 
Hi Jbreezy! :smile:

Are you allowed to use the associative rule, a x (b + c) = a x b + a x c ?

Or do you have to use coordinates?
 
I guess you are allowed to use whatever. I don't understand this problem at all.
 
ok, then what is x x (u - v) ? :wink:
 
0 vector? This is not anywhere in my eq. though. In terms of the original.
 
This was a good problem to make me feel like a monkey with a stick.

so, If x + v = u
Prove x x v = u x v = x x u

Sub in u.

x x v = (x + v ) x v = x x(x + v)

So when you distribute.

X x V = X x V + V x V = X x X + X x v

So, V x V = 0 , X x X = 0
So
X x V = X x V = X x V
Thanks
 
Hi Jbreezy! :smile:

(just got up :zzz:)

Yes, that's correct. :smile:

But it's a bit long-winded …

you could have done x x v = (x + v ) x v (because v x v = 0)

= u x v,​

or x x v - u x v = … (you finish it :wink:)
 

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