Question #101027
1. Find the vector form of the general solution of the given linear system Ax = b ; then use that result to find the vector form of the general solution of Ax = 0.

x subscript 1 + x subscript 2 + 2x subscript 3 = 6
x subscript 1 + x subscript 3 = -2
2x subscript 1 + x subscript 2 + 3x subscript = 4

2. Find a basis for the null space and row space of A.
1 -1 3
A = 5 -4 -2
7 -6 4

3. A matrix in row echelon form is given. By inspection, find bases for the row and column spaces of the matrix A.

1 0 2
A = 0 0 1
0 0 0
1
Expert's answer
2020-01-08T12:31:16-0500

1. Find the vector form of the general solution of the given linear system Ax = b ; then use that result to find the vector form of the general solution of Ax = 0.

{x1+x2+2x3=6x1+x3=22x1+x2+3x3=4\begin{cases} x_1+x_2+2x_3=6 \\x_1+x_3=-2 \\ 2x_1+x_2+3x_3=4 \end{cases}



Let x3=tx_3=t , replace this in the second equation: x1=2tx_1=-2-t

Then replace x3=t,x1=2tx_3=t, x_1=-2-t in the first and third equations:


{(2t)+x2+2t=62(2t)+x2+3t=4,x2=8t\begin{cases} (-2-t)+x_2+2t=6 \\ 2(-2-t)+x_2+3t=4 \end{cases}, \quad x_2= 8-t


The general solution of the linear system Ax=b is


[x1x2x3]=[2t8tt]=[280]+t[110],tR\begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix} = \begin{bmatrix} -2-t \\ 8-t \\t \end{bmatrix} = \begin{bmatrix} -2 \\ 8 \\0 \end{bmatrix}+ t\begin{bmatrix}-1 \\ -1 \\ 0\end{bmatrix}, \quad t\in \mathbb{R}


The general solution of the linear system Ax=0 is


[x1x2x3]=t[110],tR\begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix} = t\begin{bmatrix}-1 \\ -1 \\ 0\end{bmatrix}, \quad t\in \mathbb{R}




2. Find a basis for the null space and row space of A.

A=[113542764]A =\begin{bmatrix} 1&-1&3 \\ 5&-4&-2 \\ 7&-6&4 \end{bmatrix}


We reduce the matrix A as follows


A=[113542764]R37R1R25R1[11301170117]R1+R2R3R2[10140117000]A =\begin{bmatrix} 1&-1&3 \\ 5&-4&-2 \\ 7&-6&4 \end{bmatrix} \to ^{R_2-5R_1}_{R_3-7R_1}\to \begin{bmatrix} 1&-1&3 \\ 0&1&-17 \\ 0&1&-17 \end{bmatrix} \to^{R_3-R_2}_{R_1+R_2} \to \begin{bmatrix} 1&0&-14 \\ 0&1&-17 \\ 0&0&0 \end{bmatrix}


The last matrix is in reduced row echelon form.


a) basis for null space

We see that the general solution of Ax=0 is

{x1=14x3x2=17x3\begin{cases} x_1=14x_3 \\ x_2=17x_3 \end{cases}

where x3x_3 is free variable.

The vector form solution to Ax=0 is

[x1x2x3]=x3[14171],x3R\begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix} = x_3\begin{bmatrix}14 \\ 17 \\ 1\end{bmatrix}, \quad x_3\in \mathbb{R}


The null space of the matrix A is given by

N(A)={xR3x=x3[14171],x3R}=Span{[14171]}N(A)=\{ x\in \mathbb{R}^3 | x= x_3\begin{bmatrix}14 \\ 17 \\ 1\end{bmatrix}, \quad x_3\in \mathbb{R}\} = Span \begin{Bmatrix} {\begin{bmatrix}14 \\ 17 \\ 1\end{bmatrix} } \end{Bmatrix}


We can see, that vector [14171]\begin{bmatrix}14 \\ 17 \\ 1\end{bmatrix} is a basis for null space.


b) basis for row space

The nonzero rows of matrix (in reduced row echelon form) is a basis for the row space of A.

Thus, {[1014],[0117]}\begin{Bmatrix} \begin{bmatrix}1 \\ 0 \\ -14\end{bmatrix},\begin{bmatrix}0 \\ 1 \\ -17\end{bmatrix} \end{Bmatrix} is a basis for the row space of A.




3. A matrix in row echelon form is given. By inspection, find bases for the row and column spaces of the matrix A.

A=(102001000)A=\begin{pmatrix} 1&0&2 \\ 0&0&1 \\ 0&0&0 \end{pmatrix}



a) bases for the row space

The nonzero rows of a matrix in row echelon form are linearly independent.

Therefore, the row space has basis {[102],[001]}\begin{Bmatrix} \begin{bmatrix}1 \\ 0 \\ 2\end{bmatrix},\begin{bmatrix}0 \\ 0 \\ 1\end{bmatrix} \end{Bmatrix}

b) bases for the column space

The nonzero columns of a matrix in row echelon form are linearly independent.

Therefore, the column space has basis {[100],[210]}\begin{Bmatrix} \begin{bmatrix}1 \\ 0 \\ 0\end{bmatrix},\begin{bmatrix}2 \\ 1 \\ 0\end{bmatrix} \end{Bmatrix}





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