Exploring the Dot Product Expansion: Proving the Relationship between a and b

In summary, the dot product of vectors a and b is equal to 1/4|a+b|^2 - 1/4|a-b|^2. This can be proven by using the combination of the cosine law and the dot product equations, specifically applying the equation v dot v = |v|^2 for any vector v. By setting v = a+b and u = a-b, the expansion of (a+b) dot (a+b) can be simplified to a^2 + b^2 + 2ab, and the expansion of (a-b) dot (a-b) can be simplified to a^2 + b^2 - 2ab. The difference between these two expansions results in the dot product
  • #1
Elektrykia
4
0

Homework Statement


Show that the dot product of vectors a and b is equal to 1/4|a+b|^2 - 1/4|a-b|^2


Homework Equations


a dot b = |a||b|cos(theta)
a dot b = a1b1 + a2b2 + ...


The Attempt at a Solution


I've tried using the combination of the cosine law and those two above dot product equations and I have gotten no where. Any help would be greatly appreciated. (:
 
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  • #2
|v|^2=v dot v for any vector v. Apply that with v=a+b and v=a-b.
 
  • #3
I've tried that, maybe there's something I'm missing but I do v=a+b and u=a-b and get them to a point where i can expand them out and I get 1/4ab as the answer.
 
  • #4
The ab=a dot b is right. The 1/4 isn't right. What do you get for the expansion of (a+b) dot (a+b)?
 
  • #5
(a+b) dot (a+b) = a^2 + b^2 + 2ab, yes?
 
  • #6
Elektrykia said:
(a+b) dot (a+b) = a^2 + b^2 + 2ab, yes?

Yes. 'ab' means 'a dot b', correct? And (a-b) dot (a-b)? And the difference between the two?
 
  • #7
Haha, oh wow, I looked over that like four times.
For some reason ab looked so wrong to me when I got it, I understand it now.

Thanks very much (:
 

1. What is the dot product expansion?

The dot product expansion is a mathematical operation that calculates the scalar product of two vectors. It involves multiplying corresponding components of the vectors and then adding the results. It is often used in physics and engineering to find the work done by a force or the angle between two vectors.

2. How is the dot product expansion calculated?

The dot product expansion is calculated by multiplying the first component of one vector with the first component of the other vector, then multiplying the second component of one vector with the second component of the other vector, and so on. The results of these multiplications are then added together to get the scalar product.

3. What is the difference between the dot product expansion and cross product?

The dot product expansion and cross product are both mathematical operations involving vectors, but they have different results. The dot product expansion results in a scalar quantity, while the cross product results in a vector quantity. Additionally, the dot product expansion is commutative (order of vectors does not matter), while the cross product is anti-commutative (order matters).

4. What are some applications of the dot product expansion?

The dot product expansion has many applications in physics and engineering. It is used to calculate the work done by a force, the angle between two vectors, and the projection of one vector onto another. It is also used in geometry to find the angle between two lines or planes.

5. Can the dot product expansion be used for vectors in any dimension?

Yes, the dot product expansion can be used for vectors in any dimension. It is a general mathematical operation that can be applied to vectors with any number of components. However, the resulting scalar product will only have a physical meaning in two or three dimensions.

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