- 3 x₂ + 7 x₃ = 3
x₁ + 2 x₂ - x₃ = 0
5 x₁ - 2 x₂ = 3
(a) I suppose this is asking for the determinant of the coefficient matrix.
[tex]\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}[/tex]
Using a cofactor expansion along the first row, this reduces to
[tex]0\begin{vmatrix}2&-1\\-2&0\end{vmatrix}-(-3)\begin{vmatrix}1&-1\\5&0\end{vmatrix}+7\begin{vmatrix}1&2\\5&-2\end{vmatrix}= 3(0 - (-5))+7(-2-10)=\boxed{-69}[/tex]
(b) Using Cramer's rule, we have
[tex]x_1=\dfrac{\begin{vmatrix}\mathbf 3&-3&7\\\mathbf 0&2&-1\\\mathbf 3&-2&0\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]
[tex]x_2=\dfrac{\begin{vmatrix}0&\mathbf 3&7\\1&\mathbf 0&-1\\5&\mathbf 3&0\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]
[tex]x_3=\dfrac{\begin{vmatrix}0&-3&\mathbf 3\\1&2&\mathbf 0\\5&-2&\mathbf 3\end{vmatrix}}{\begin{vmatrix}0&-3&7\\1&2&-1\\5&-2&0\end{vmatrix}}[/tex]
That is, solving for n-th variable consists of dividing [the determinant of the coefficient matrix with its n-th column replaced with the right side of the system, the numbers in boldface,] by [the determinant of the coefficient matrix].
Compute each determinant:
[tex]\begin{vmatrix}\mathbf 3&-3&7\\\mathbf 0&2&-1\\\mathbf 3&-2&0\end{vmatrix}=3\begin{vmatrix}2&-1\\-2&0\end{vmatrix}+3\begin{vmatrix}-3&7\\2&-1\end{vmatrix}=3(0-2)+3(3-14)=-39[/tex]
(expanding along the first column)
[tex]\begin{vmatrix}0&\mathbf 3&7\\1&\mathbf 0&-1\\5&\mathbf 3&0\end{vmatrix}=-1\begin{vmatrix}3&7\\3&0\end{vmatrix}+5\begin{vmatrix}3&7\\0&-1\end{vmatrix}=-1(0-21)+5(-3-0)=6[/tex]
(again, along the first column)
[tex]\begin{vmatrix}0&-3&\mathbf 3\\1&2&\mathbf 0\\5&-2&\mathbf 3\end{vmatrix}=-1\begin{vmatrix}-3&3\\-2&3\end{vmatrix}+5\begin{vmatrix}-3&3\\2&0\end{vmatrix}=-1(-9-(-6))+5(0-6)=-27[/tex]
(first column)
So, we get the solution
[tex]x_1=\dfrac{-39}{-69}=\boxed{\dfrac{13}{23}},x_2=\dfrac6{-69}=\boxed{-\dfrac2{23}},x_3=\dfrac{-27}{-69}=\boxed{\dfrac9{23}}[/tex]
(c) Using elimination:
- 3 x₂ + 7 x₃ = 3
x₁ + 2 x₂ - x₃ = 0
5 x₁ - 2 x₂ = 3
Swap the first two equations:
x₁ + 2 x₂ - x₃ = 0
- 3 x₂ + 7 x₃ = 3
5 x₁ - 2 x₂ = 3
Add -5(equation 1) to equation 3:
x₁ + 2 x₂ - x₃ = 0
- 3 x₂ + 7 x₃ = 3
- 12 x₂ + 5 x₃ = 3
Add -4(equation 2) to equation 3:
x₁ + 2 x₂ - x₃ = 0
- 3 x₂ + 7 x₃ = 3
- 23 x₃ = -9
Multiply through equation 3 by -1/23:
x₁ + 2 x₂ - x₃ = 0
- 3 x₂ + 7 x₃ = 3
x₃ = 9/23
Add -7(equation 3) to equation 2:
x₁ + 2 x₂ - x₃ = 0
- 3 x₂ = 6/23
x₃ = 9/23
Multiply through equation 2 by -1/3:
x₁ + 2 x₂ - x₃ = 0
x₂ = -2/23
x₃ = 9/23
Add -2(equation 2) and equation 3 to equation 1:
x₁ = 13/23
x₂ = -2/23
x₃ = 9/23
