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Given A m $\times$ n is a matrix, which of the following conditions must be fulfilled so that the system of equations Ax=b has a solution for every b $\epsilon$ R$^m$

A) Every 2 columns in A are linearly independent.

B) All the columns in A are linearly independent.

C) Every 2 rows in A are linearly independent.

D) All the rows in A are linearly independent.

E) The columns in A span R$^m$.

F) The rows in A span R$^n$.

G) All columns in A are not zeroes.

H) All rows in A are not zeroes.

I am certain that A has to be a linearly independent square matrix according to Cramer's rule, which would mean the determinant of A $\not$= 0, and that m=n.

So I believe statements B, E, H are true. However, that is not the case apparently.

What am I missing here?

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  • $\begingroup$ $A$ need not be a square matrix, for example take $\begin{bmatrix}1&1&0\end{bmatrix}\begin{bmatrix}x_1\\x_2\\x_3\end{bmatrix}=b$. It has a solution for every $b \in \mathbb{R^1}$. This example can help you eliminate some other possibilities in the list as well. $\endgroup$ Commented May 16, 2020 at 19:20

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A) is obviously false. Assume you already have a matrix $A$ such there there is a solution $x$ of $Ax=b$ for each $b\in\mathbb{R}^m.$ Then you could "extend" the matrix to the right with copies of columns of itself and still easily find solutions of the new linear system of equations.

B) is false for the same reason.

C) is true. It is implied by D)

D) is true. Assume the rows were dependent. Then there would be $u\in\mathbb{R}^m,\;\;u\neq 0$ such that $u^TA=0.$ If we can find a solution for every $b\in\mathbb{R}^m,$ then we can find a solution for $b=u.$ So let us assume there was an $x$ that satisfies $Ax=u.$ We would have $0=0x=u^TAx=u^Tu\neq 0.$ This is a contradiction, the rows must be linear independent.

E) is true. It is just another way of saying that for each $b\in\mathbb{R}^m,$ there exists an $x\in\mathbb{R}^n$ such that $Ax=b.$

F) is false. The subspace of $\mathbb{R}^n$ spanned by the rows of $A$ is at most $m$-dimensional (as we have only $m$ rows). And we have already seen that we can have $n>m.$

For G) and H), I assume that "columns/rows are not zeroes" means "columns/rows do not consist of zeros, only"

G) is false. Similar to A). I can append arbitrarily many columns full of zeroes to a matrix $A$ that "works" and still find a solution for each $b.$

H) is true. If I had a row full of zeroes in $A$, then the result of $Ax$ would also be $0$ in the same row. This would limit the possibility to get all possible $b.$

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