Understanding Why (A+B)(A+B) is Not Valid for Matrices: Linear Algebra Homework

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SUMMARY

The expression (A+B)(A+B) is not valid for matrices because matrix multiplication is noncommutative. The correct expansion is (A+B)(A+B) = A^2 + AB + BA + B^2, which differs from the algebraic expansion A^2 + 2AB + B^2 applicable to real numbers. The distinction arises because the order of multiplication matters in matrix operations, as demonstrated by the example of linear transformations where T(F(x)) is not equal to F(T(x)).

PREREQUISITES
  • Understanding of matrix multiplication and its properties
  • Familiarity with linear transformations and their matrix representations
  • Knowledge of noncommutative algebra concepts
  • Basic principles of linear algebra
NEXT STEPS
  • Study the properties of matrix multiplication in detail
  • Explore the concept of linear transformations and their implications
  • Learn about noncommutative algebra and its applications
  • Review examples of matrix operations that illustrate noncommutativity
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Students studying linear algebra, educators teaching matrix theory, and anyone seeking to deepen their understanding of matrix operations and linear transformations.

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Homework Statement


Explain why the formula is not valid for matrices.
(A+B)(A+B)=A^2 + 2AB + B^2


Homework Equations


none.


The Attempt at a Solution



I don't know really know how to start this. I don't really know why that is not valid. Please help me understand. Thank you.
 
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(A+B)(A+B) = A^2 + AB + BA + B^2. That much is true for matrices as well as real numbers. What goes wrong in between this line and (A+B)(A+B) = A^2 + 2 AB + B^2?
 
remember matrix multiplication is not like normal multiplication, in general it is noncommutative which means it matters which side you multiply on. another example of noncommutative algebra is the curl or cross product of vectors. If you are discussing the composition of linear transformations remember that when you multiply the matrix representation of a linear transf. it is analagous to composition of functions. obviously T(F(x)) not equal to F(T(x)) for all F,T.
 

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