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(a+b)^2=a^2+b^2 Matrix Find A And B

4 min read Jun 16, 2024
(a+b)^2=a^2+b^2 Matrix Find A And B

Finding Matrices A and B given (A+B)^2 = A^2 + B^2

This problem explores a special case in matrix algebra where the square of the sum of two matrices equals the sum of their squares. It's important to understand that this doesn't hold true for all matrices, unlike in regular algebra where (a + b)² = a² + 2ab + b².

The Key Insight:

The condition (A + B)² = A² + B² implies that the cross-product terms (AB + BA) must vanish:

(A + B)² = (A + B)(A + B) = A² + AB + BA + B²

Therefore, AB + BA = 0.

Finding A and B:

To find A and B, we can utilize the following steps:

  1. Choose a matrix structure: Start by assuming a specific structure for either A or B. This can be a diagonal matrix, a scalar multiple of the identity matrix, or any other structure that simplifies the calculations.

  2. Apply the condition AB + BA = 0: Substitute the chosen matrix structure and solve the resulting matrix equation for the remaining matrix.

  3. Check for consistency: Ensure that the found matrices satisfy the original condition (A + B)² = A² + B².

Example:

Let's assume A is a diagonal matrix:

A =

[a  0]
[0  b]

Now, we need to find B that satisfies AB + BA = 0.

Since A is diagonal, AB and BA will be:

AB =

[a*b11  a*b12]
[b*b21  b*b22]

BA =

[a*b11  b*b12]
[a*b21  b*b22]

For AB + BA = 0, we need:

  • ab12 + bb12 = 0
  • ab21 + bb21 = 0

These equations imply that either a + b = 0 or b12 = b21 = 0.

Solution:

One possible solution is:

  • **A = [a 0] and B = [-a 0] **
  • [0 b] [0 -b]

Important Considerations:

  • This problem doesn't have a unique solution. Many different matrices can satisfy the condition (A + B)² = A² + B².
  • The choice of matrix structure for A or B can significantly influence the complexity of the solution.
  • The condition AB + BA = 0 is a powerful tool to explore matrix relationships and derive specific matrix structures.

By applying these steps and exploring different matrix structures, you can find numerous solutions to the problem (A + B)² = A² + B². This problem demonstrates the unique properties of matrix algebra and provides a fascinating exercise in understanding matrix multiplication and its implications.

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