# Matrices

#### Yankel

##### Active member
Hello

I have been trying to solve a couple of true / false questions, and I am not sure my answers are correct, I would appreciate it if you could verify it.

The first question is:

A and B are matrices such that it is possible to calculate:

$C=AB+B^{t}A^{t}$

a. A and B are of the same order
b. C is symmetric
c. BA can be calculated
d. A and B are squared matrices
e. $ABA^{t}$ can be calculated

My answers are: a - false, b - true , c - true, d - false e - false

The second question is:

A is a 3x3 not invertible matrix:

a. The system Ax=b has a unique solution for every vector b
b. The system Ax=b has infinite number of solutions for every vector b
c. The matrix kA is not invertible for every real number k.
d. If rank(A)=1, then A has at least one row of 0's.
e. There exist a vector b such that Ax=b has infinite number of solutions.

My answers are: a - false, b - false, c - true, d - true, e - true

Is there something wrong with my solutions ?

Thanks a million !

#### Jameson

Staff member
Let's say $A$ is an $(m \times n)$ matrix and $B$ is $(n \times p)$. Then $AB$ is an $(m \times p)$ matrix and and $B^T A^T$ is $(p \times m)$. This implies that $p=m$ for it to be possible to add the two products together to make $C$ but that doesn't mean that $A$ and $B$ are necessarily of the same order. It seems to me that the two matrices could be square but don't have to be.

Assuming my reasoning is correct then your answers for the first one look good. They are all consistent too, which is good.

#### Opalg

##### MHB Oldtimer
Staff member
Hello

I have been trying to solve a couple of true / false questions, and I am not sure my answers are correct, I would appreciate it if you could verify it.

The first question is:

A and B are matrices such that it is possible to calculate:

$C=AB+B^{t}A^{t}$

a. A and B are of the same order
b. C is symmetric
c. BA can be calculated
d. A and B are squared matrices
e. $ABA^{t}$ can be calculated

My answers are: a - false, b - true , c - true, d - false e - false

The second question is:

A is a 3x3 not invertible matrix:

a. The system Ax=b has a unique solution for every vector b
b. The system Ax=b has infinite number of solutions for every vector b
c. The matrix kA is not invertible for every real number k.
d. If rank(A)=1, then A has at least one row of 0's.
e. There exist a vector b such that Ax=b has infinite number of solutions.

My answers are: a - false, b - false, c - true, d - true, e - true

Is there something wrong with my solutions ?

Thanks a million !
I would take a second look at 2d. Everything else looks good.

#### Yankel

##### Active member
you are right, my mistake !

If rank(A)=1, the matrix after the elementary row operations has at least one 0 row, not the original A.

Thanks !