Assume that the product AB makes sense. Prove that if the columns of B are linea

rabbitz42z8

rabbitz42z8

Answered question

2021-11-08

Assume that the product AB makes sense. Prove that if the columns of B are linearly dependent, then so are the columns of AB.

Answer & Explanation

Befoodly

Befoodly

Beginner2021-11-09Added 17 answers

Let 
B=[b1 b2  bn] 
where b1,b2,,bn are columns of the matrix B. According to our hypothesis those columns are linearly dependent, so there are some constants c1,c2,cn such that c1b1+c2b2++cnbn=0  and  ci0 for some i{1,2,,n}. The product of 
matrices A and B is then: 
AB=[Ab1 Ab2  Abn] 
where the ith column of the matrix AB is product of the matrix A and the ith column of the matrix B. Now we have that: 
c1Ab1+c2Ab2++cnAbn=Ac1b1+Ac2b2++Acnbn= 
=A(c1b1+c2+b2++cnbn)= 
=A(0)=0. 
There exist constants c1,c2,,cn such that c1Ab1+c2Ab2++cnAbn=0  and  ci0 for some i{1,2,,n}, so columns of the matrix AB are linearly dependent. 
Results: 
Due to the linear dependence of the columns of the matrix B, there are non-trivial constants that guarantee that the result of a linear combination of those columns is 0. Show that the columns of matrix AB make a linear combination equal to 0, indicating that they are linearly dependent, by using the same constants.

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