Dot product between cross products

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SUMMARY

The discussion focuses on proving the equation (a × b) · (c × d) = |a·c b·c| |a·d b·d|. Participants clarify that the right side represents the determinant of a matrix formed by the dot products of vectors a, b, c, and d. The use of the Levi-Civita symbol is suggested as a method to simplify the cross product calculations, although it is noted that it is not mandatory for the proof. The proof can be approached in three steps, emphasizing the distribution law in vector operations.

PREREQUISITES
  • Understanding of vector operations, specifically cross products and dot products.
  • Familiarity with determinants and matrix representation of vectors.
  • Knowledge of the Levi-Civita symbol and its application in vector calculus.
  • Basic principles of linear algebra and vector spaces.
NEXT STEPS
  • Study the properties of the Levi-Civita symbol in vector calculus.
  • Learn how to compute determinants of 2x2 matrices.
  • Explore the distribution law in vector algebra.
  • Practice proving vector identities involving cross and dot products.
USEFUL FOR

Students of linear algebra, physics enthusiasts, and anyone looking to deepen their understanding of vector calculus and its applications in mathematical proofs.

quietrain
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Homework Statement


show (axb) . (cxd) =
|a.c b.c|
|a.d b.d|

The Attempt at a Solution



i have no idea. i don't know if the lines at the side are modulus lines or matrix brackets

but it seems that it has something to do with distribution law.

but i can't start proving if i don't even understand what the right hand side of the equation is ><

thanks for help!
 
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Right side is a determinant of this matrix. It's not so hard to prove, you could in example use Levi-Civita symbol to write your cross product, it goes in 3 steps afterwards.
 
irycio said:
Right side is a determinant of this matrix. It's not so hard to prove, you could in example use Levi-Civita symbol to write your cross product, it goes in 3 steps afterwards.

oh i have done it. but i didn't use that symbol? thanks anyway!
 
Well, you don't have to use it, that's just my favourite way to deal with cross product stuff :).
 

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