Step 1: Write the augmented matrix for the system A x = c Ax = \mathbf{c} A x = c .
[ 6 2 4 4 1 4 3 − 2 − 2 9 3 1 ] \left[
\begin{array}{ccc|c}
6 & 2 & 4 & 4 \\
1 & 4 & 3 & -2 \\
-2 & 9 & 3 & 1
\end{array}
\right] 6 1 − 2 2 4 9 4 3 3 4 − 2 1
Step 2: Swap row 1 (R1) and row 2 (R2) to get a leading 1 in position (1,1).
[ 1 4 3 − 2 6 2 4 4 − 2 9 3 1 ] \left[
\begin{array}{ccc|c}
1 & 4 & 3 & -2 \\
6 & 2 & 4 & 4 \\
-2 & 9 & 3 & 1
\end{array}
\right] 1 6 − 2 4 2 9 3 4 3 − 2 4 1
Step 3: Eliminate column 1 below the pivot: R2 ← \leftarrow ← R2 − 6 - 6 − 6 R1; R3 ← \leftarrow ← R3 + 2 + 2 + 2 R1.
R2: [ 6 − 6 ( 1 ) , 2 − 6 ( 4 ) , 4 − 6 ( 3 ) ∣ 4 − 6 ( − 2 ) ] = [ 0 , − 22 , − 14 ∣ 16 ] [6-6(1),\ 2-6(4),\ 4-6(3)\ |\ 4-6(-2)] = [0,\ -22,\ -14\ |\ 16] [ 6 − 6 ( 1 ) , 2 − 6 ( 4 ) , 4 − 6 ( 3 ) ∣ 4 − 6 ( − 2 )] = [ 0 , − 22 , − 14 ∣ 16 ]
R3: [ − 2 + 2 ( 1 ) , 9 + 2 ( 4 ) , 3 + 2 ( 3 ) ∣ 1 + 2 ( − 2 ) ] = [ 0 , 17 , 9 ∣ − 3 ] [-2+2(1),\ 9+2(4),\ 3+2(3)\ |\ 1+2(-2)] = [0,\ 17,\ 9\ |\ -3] [ − 2 + 2 ( 1 ) , 9 + 2 ( 4 ) , 3 + 2 ( 3 ) ∣ 1 + 2 ( − 2 )] = [ 0 , 17 , 9 ∣ − 3 ]
[ 1 4 3 − 2 0 − 22 − 14 16 0 17 9 − 3 ] \left[
\begin{array}{ccc|c}
1 & 4 & 3 & -2 \\
0 & -22 & -14 & 16 \\
0 & 17 & 9 & -3
\end{array}
\right] 1 0 0 4 − 22 17 3 − 14 9 − 2 16 − 3
Step 4: Simplify row 2: Divide R2 by − 2 -2 − 2 to get R2 ← − 1 2 \leftarrow -\frac{1}{2} ← − 2 1 R2.
[ 0 , 11 , 7 ∣ − 8 ] [0,\ 11,\ 7\ |\ -8] [ 0 , 11 , 7 ∣ − 8 ]
[ 1 4 3 − 2 0 11 7 − 8 0 17 9 − 3 ] \left[
\begin{array}{ccc|c}
1 & 4 & 3 & -2 \\
0 & 11 & 7 & -8 \\
0 & 17 & 9 & -3
\end{array}
\right] 1 0 0 4 11 17 3 7 9 − 2 − 8 − 3
Step 5: Eliminate column 2 below the pivot: Multiply R2 by 17 and R3 by 11, then subtract.
17 × \times × R2: [ 0 , 187 , 119 ∣ − 136 ] [0,\ 187,\ 119\ |\ -136] [ 0 , 187 , 119 ∣ − 136 ]
11 × \times × R3: [ 0 , 187 , 99 ∣ − 33 ] [0,\ 187,\ 99\ |\ -33] [ 0 , 187 , 99 ∣ − 33 ]
New R3 ← \leftarrow ← 17R2 − - − 11R3: [ 0 , 0 , 20 ∣ − 103 ] [0,\ 0,\ 20\ |\ -103] [ 0 , 0 , 20 ∣ − 103 ]
[ 1 4 3 − 2 0 11 7 − 8 0 0 20 − 103 ] \left[
\begin{array}{ccc|c}
1 & 4 & 3 & -2 \\
0 & 11 & 7 & -8 \\
0 & 0 & 20 & -103
\end{array}
\right] 1 0 0 4 11 0 3 7 20 − 2 − 8 − 103
Step 6: Back-substitute starting from the bottom.
From R3: 20 x 3 = − 103 ⟹ x 3 = − 103 20 20x_3 = -103 \implies x_3 = -\frac{103}{20} 20 x 3 = − 103 ⟹ x 3 = − 20 103
Substitute into R2: 11 x 2 + 7 ( − 103 20 ) = − 8 11x_2 + 7\left(-\frac{103}{20}\right) = -8 11 x 2 + 7 ( − 20 103 ) = − 8
11 x 2 − 721 20 = − 8 11x_2 - \frac{721}{20} = -8 11 x 2 − 20 721 = − 8
11 x 2 = − 8 + 721 20 = − 160 20 + 721 20 = 561 20 11x_2 = -8 + \frac{721}{20} = -\frac{160}{20} + \frac{721}{20} = \frac{561}{20} 11 x 2 = − 8 + 20 721 = − 20 160 + 20 721 = 20 561
x 2 = 561 20 × 11 = 561 220 = 51 20 x_2 = \frac{561}{20 \times 11} = \frac{561}{220} = \frac{51}{20} x 2 = 20 × 11 561 = 220 561 = 20 51
Substitute into R1: x 1 + 4 ( 51 20 ) + 3 ( − 103 20 ) = − 2 x_1 + 4\left(\frac{51}{20}\right) + 3\left(-\frac{103}{20}\right) = -2 x 1 + 4 ( 20 51 ) + 3 ( − 20 103 ) = − 2
x 1 + 204 20 − 309 20 = − 2 x_1 + \frac{204}{20} - \frac{309}{20} = -2 x 1 + 20 204 − 20 309 = − 2
x 1 − 105 20 = − 2 x_1 - \frac{105}{20} = -2 x 1 − 20 105 = − 2
x 1 = − 2 + 105 20 = − 40 20 + 105 20 = 65 20 = 13 4 x_1 = -2 + \frac{105}{20} = -\frac{40}{20} + \frac{105}{20} = \frac{65}{20} = \frac{13}{4} x 1 = − 2 + 20 105 = − 20 40 + 20 105 = 20 65 = 4 13
The solution is x = [ x 1 x 2 x 3 ] = [ 13 4 51 20 − 103 20 ] \mathbf{x} = \begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix} = \begin{bmatrix} \frac{13}{4} \\ \frac{51}{20} \\ -\frac{103}{20} \end{bmatrix} x = x 1 x 2 x 3 = 4 13 20 51 − 20 103 .
\begin{bmatrix \dfrac{13}{4} \\[1em] \dfrac{51}{20} \\[1em] -\dfrac{103}{20} \end{bmatrix}}