MHB Jamal's Q via email solving a system

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The discussion outlines the process of solving a system of equations using Gaussian elimination with partial pivoting, represented in an augmented matrix. The elimination steps are detailed, showing how to transform the matrix into an upper triangular form while performing row operations. The final augmented matrix allows for back substitution to find the values of the variables. The solution to the system is found to be x = [2, -1, 0, 3]. Verifying the solution against the original matrix confirms its accuracy.
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We can write the system in an augmented matrix as $\displaystyle \begin{align*} A = \left[ \begin{matrix} 2 & -2 & -1 & \phantom{-}0 & \phantom{-}6 \\ 2 & \phantom{-}3 & \phantom{-}2 & -5 &-14 \\ 0 & \phantom{-}5 & \phantom{-}2 & \phantom{-}2 &\phantom{-}1 \\ 0 & \phantom{-}0 & \phantom{-}2 & -3 & -9 \end{matrix} \right] \end{align*}$. If we apply Gaussian Elimination with Partial Pivoting, when the system is upper-triangularised, the right hand column becomes the elements of matrix $\displaystyle \begin{align*} \mathbf{g} \end{align*}$.

First we will apply R2 - R1 to R1 (no pivoting needed as the elements in the first column have the same magnitude)

$\displaystyle \begin{align*} \left[ \begin{matrix} 2 & -2 & -1 & \phantom{-}0 &\phantom{-}6 \\ 0 & \phantom{-}5 & \phantom{-}3 & -5 &-20 \\ 0 & \phantom{-}5 & \phantom{-}2 & \phantom{-}2 &\phantom{-}1 \\ 0 &\phantom{-}0 & \phantom{-}2 & -3 &-9 \end{matrix} \right] \end{align*}$

Again pivoting is not needed as the elements on or below the main diagonal in column 2 are the same magnitude, so we will apply R3 - R2 to R2 giving

$\displaystyle \begin{align*} \left[ \begin{matrix} 2 & -2 & -1 & \phantom{-}0 &\phantom{-}6 \\ 0 & \phantom{-}5 & \phantom{-}3 & -5 &-20 \\ 0 & \phantom{-}0 & -1 & \phantom{-}7 &\phantom{-}21 \\ 0 &\phantom{-}0 & \phantom{-}2 & -3 &-9 \end{matrix} \right] \end{align*}$

If we look in column 3, we see that the element on or below the main diagonal with the highest magnitude is in Row 4, so we must switch rows 3 and 4, giving

$\displaystyle \begin{align*} \left[ \begin{matrix} 2 & -2 & -1 & \phantom{-}0 &\phantom{-}6 \\ 0 & \phantom{-}5 & \phantom{-}3 & -5 &-20 \\ 0 &\phantom{-}0 & \phantom{-}2 & -3 &-9\\ 0 & \phantom{-}0 & -1 & \phantom{-}7 &\phantom{-}21 \end{matrix} \right] \end{align*}$

and finally when we apply R4 + (1/2)R3 to R4 we get

$\displaystyle \begin{align*} \left[ \begin{matrix} 2 & -2 & -1 & \phantom{-}0 &\phantom{-}6 \\ 0 & \phantom{-}5 & \phantom{-}3 & -5 &-20 \\ 0 &\phantom{-}0 & \phantom{-}2 & -3 &-9\\ 0 & \phantom{-}0 & \phantom{-}0 & \phantom{-}\frac{11}{2} & \phantom{-}\frac{33}{2} \end{matrix} \right] \end{align*}$So we can read off that $\displaystyle \begin{align*} \mathbf{g} = \left[ \begin{matrix} \phantom{-}6 \\ -20 \\ -9 \\ \phantom{-}\frac{33}{2} \end{matrix} \right] \end{align*}$ and we can solve for $\displaystyle \begin{align*} \mathbf{x} \end{align*}$ in the system through back substitution:

$\displaystyle \begin{align*} \frac{11}{2} \, x_4 &= \frac{33}{2} \\ x_4 &= 3 \\ \\ 2\,x_3 - 3\,x_4 &= -9 \\ 2\,x_3 - 9 &= -9 \\ 2\,x_3 &= 0 \\ x_3 &= 0 \\ \\ 5\,x_2 + 3\,x_3 - 5\,x_4 &= -20 \\ 5\,x_2 + 0 - 15 &= -20 \\ 5\,x_2 &= -5 \\ x_2 &= -1 \\ \\ 2\,x_1 - 2\,x_2 - x_3 &= 6 \\ 2\,x_1 + 2 - 0 &= 6 \\ 2\,x_1 &= 4 \\ x_1 &= 2 \end{align*}$

Thus the solution to the system is $\displaystyle \begin{align*} \mathbf{x} = \left[ \begin{matrix} \phantom{-}2 \\ -1 \\ \phantom{-}0 \\ \phantom{-}3 \end{matrix} \right] \end{align*}$.
 
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As always, it's a good idea to check that the solution shown above actually works in the augmented matrix at the beginning of the post.
 
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