Proof this cross product property

In summary, the homework statement is trying to show that:- a x b = ||a|| ||b|| sin x- (a.b)^2 = (a.b)^2 - (a.b)^2- (a x b)^2 = (a.b)^2
  • #1
-EquinoX-
564
1

Homework Statement


Show that :

|| a x b||^2 = ||a||^2||b||^2 - (a . b)^2

Homework Equations


The Attempt at a Solution



I don't know where to start off
 
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  • #2
What's the difference between ||axb||^2 and (axb)^2? Don't you mean (a.b)^2 instead of (axb)^2? Just use expressions for the magnitude of the cross and dot product and the angle between the vectors a and b.
 
  • #3
What you're supposed to show doesn't make sense.
From your title I assume you're asking about the vector cross product for vectors in R^3.
Given that, I understand what is meant by || a x b||^2, ||a||^2, and ||b||^2, but I have no idea what (a x b)^2 even means, let alone how to compute it.

Can you clarify what is meant here by (a x b)^2?
 
  • #4
sorry a typo, I've revised the question, it should be (a.b)^2

I know that a x b = ||a|| ||b|| sin x, so then how do I find the magnitude of this? I am confused because there's the sin function...
 
  • #5
Then express both side in terms of |a|, |b| and the angle between them.
 
  • #6
not sure I understand what you mean
 
  • #7
If t is the angle between a and b, then a.b=|a|*|b|*cos(t). There a similar relation for |axb|.
 
  • #8
ok is this correct?

[tex]|a\times b|^2 = |a|^2|b|^2\sin^2\theta = |a|^2|b|^2(1 - \cos^2\theta)[/tex]
[tex] = |a|^2|b|^2 - |a|^2|b|^2\cos^2\theta = |a|^2|b|^2 - (a\cdot b)^2 [/tex]
 
  • #9
-EquinoX- said:
ok is this correct?

[tex]|a\times b|^2 = |a|^2|b|^2\sin^2\theta = |a|^2|b|^2(1 - \cos^2\theta)[/tex]
[tex] = |a|^2|b|^2 - |a|^2|b|^2\cos^2\theta = |a|^2|b|^2 - (a\cdot b)^2 [/tex]

Absolutely.
 
  • #10
I am not sure of the notations through can I write |a x b|^2 as ||a x b||^2 as it denotes magnitude?
 
  • #11
They both mean the same thing to me. If you're a stickler you might want to demand writing ||v|| if v is a vector and |x| if x is a number.
 
  • #12
well yes, but what's meant here by ||v|| here is the magnitude of vector v right? now I am confused though, by definition of the geometric, it says that:

a x b = ||a|| ||b|| sin x

and it's not

|a x b| = ||a|| ||b|| sin x

can you explain this?
 
  • #13
-EquinoX- said:
well yes, but what's meant here by ||v|| here is the magnitude of vector v right? now I am confused though, by definition of the geometric, it says that:

a x b = ||a|| ||b|| sin x

and it's not

|a x b| = ||a|| ||b|| sin x

can you explain this?

I don't understand what you're saying above. Does "and it's not" refer to the first equation or the second? Either way, the first equation is incorrect and the second one is correct (aside from inconsistent usage of | | and || ||).

a X b is a vector, which ||a|| ||b|| sin x is the product of three numbers, and so is a number.
 
  • #14
Can you please prove for me this identity: A x (B x C) = (A.C)B - (A.B)C
 
  • #15
Can you please prove for me this identity, analytically: A x (B x C) = (A.C)B - (A.B)C and llU x Vll^2 = llUll^2 llVll^2 - (U.V)^2. Any one with understanding on this please help out.
 

FAQ: Proof this cross product property

1. What is the cross product property?

The cross product property is a mathematical rule that defines the relationship between two vectors in three-dimensional space. It states that the cross product of two vectors will result in a new vector that is perpendicular to both of the original vectors.

2. How is the cross product property useful?

The cross product property is useful in many areas of science and engineering, such as physics, mechanics, and computer graphics. It allows us to calculate the direction and magnitude of a new vector based on the orientation of two other vectors.

3. How is the cross product calculated?

The cross product is calculated using the formula: a x b = |a||b|sin(θ)n, where a and b are the two original vectors, |a| and |b| are their magnitudes, θ is the angle between them, and n is a unit vector perpendicular to both a and b.

4. Can the cross product property be applied to vectors in any dimension?

No, the cross product property only applies to vectors in three-dimensional space. In higher dimensions, a similar concept called the wedge product is used instead.

5. How is the cross product property proven?

The cross product property can be proven using geometric and algebraic methods. One proof involves using the dot product to show that the cross product is orthogonal to both original vectors. Another proof involves using the determinant of a matrix representation of the cross product to show its properties.

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