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

In summary, the formula (A+B)(A+B)=A^2 + 2AB + B^2 is not valid for matrices because matrix multiplication is not commutative, meaning that the order in which you multiply matrices matters. This is similar to other noncommutative algebraic operations, such as the curl or cross product of vectors. When discussing the composition of linear transformations using matrix multiplication, it is important to remember that it is analogous to composition of functions, where the order matters.
  • #1
ephemeral1
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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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  • #2
(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?
 
  • #3
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.
 

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

Why is (A+B)(A+B) not valid for matrices in linear algebra?

According to the rules of matrix multiplication, the number of columns in the first matrix must match the number of rows in the second matrix. In the case of (A+B)(A+B), the first matrix has two columns and the second matrix has two rows, which does not satisfy this rule.

Can (A+B)(A+B) ever be valid for matrices?

Yes, it can be valid for matrices if and only if the number of columns in the first matrix is equal to the number of rows in the second matrix. This is known as the commutative property of multiplication.

What is the correct way to multiply two matrices in linear algebra?

The correct way to multiply two matrices is to ensure that the number of columns in the first matrix is equal to the number of rows in the second matrix. The resulting matrix will have the same number of rows as the first matrix and the same number of columns as the second matrix.

What is the purpose of understanding why (A+B)(A+B) is not valid for matrices?

Understanding why (A+B)(A+B) is not valid for matrices is important in order to perform accurate calculations and avoid errors in linear algebra. It also helps to develop a deeper understanding of the rules and properties of matrix multiplication.

Can the order of multiplication be changed in matrix multiplication?

Yes, the order of multiplication can be changed in matrix multiplication. This is known as the associative property of multiplication. However, the resulting matrix will still have the same dimensions as the original matrices.

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