Linear Algebra Examples

$y = 36 - 9 x$ , $3 x + \frac{y}{3} = 12$

Step 1

Find the $A X = B$ from the system of equations.

$[\begin{array}{c} 9 & 1 \\ 3 & \frac{1}{3} \end{array}] \cdot [\begin{array}{c} x \\ y \end{array}] = [\begin{array}{c} 36 \\ 12 \end{array}]$

Step 2

Find the inverse of the coefficient matrix.

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The inverse of a $2 \times 2$ matrix can be found using the formula $\frac{1}{| A |} [\begin{array}{c} d & - b \\ - c & a \end{array}]$ where $| A |$ is the determinant of $A$ .

If $A = [\begin{array}{c} a & b \\ c & d \end{array}]$ then $A^{- 1} = \frac{1}{| A |} [\begin{array}{c} d & - b \\ - c & a \end{array}]$

Find the determinant of $[\begin{array}{c} 9 & 1 \\ 3 & \frac{1}{3} \end{array}]$ .

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These are both valid notations for the determinant of a matrix.

$determinant [\begin{array}{c} 9 & 1 \\ 3 & \frac{1}{3} \end{array}] = | \begin{array}{c} 9 & 1 \\ 3 & \frac{1}{3} \end{array} |$

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$(9) (\frac{1}{3}) - 3 \cdot 1$

Simplify the determinant.

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Simplify each term.

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Cancel the common factor of $3$ .

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Factor $3$ out of $9$ .

$3 (3) \frac{1}{3} - 3 \cdot 1$

Cancel the common factor.

$3 \cdot 3 \frac{1}{3} - 3 \cdot 1$

Rewrite the expression.

$3 - 3 \cdot 1$

Multiply $- 3$ by $1$ .

$3 - 3$

Subtract $3$ from $3$ .

$0$

Substitute the known values into the formula for the inverse of a matrix.

$\frac{1}{0} [\begin{array}{c} \frac{1}{3} & - (1) \\ - (3) & 9 \end{array}]$

Simplify each element in the matrix.

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Rearrange $- (1)$ .

$\frac{1}{0} [\begin{array}{c} \frac{1}{3} & - 1 \\ - (3) & 9 \end{array}]$

Rearrange $- (3)$ .

$\frac{1}{0} [\begin{array}{c} \frac{1}{3} & - 1 \\ - 3 & 9 \end{array}]$

Multiply $\frac{1}{0}$ by each element of the matrix.

$[\begin{array}{c} \frac{1}{0} \cdot \frac{1}{3} & \frac{1}{0} \cdot - 1 \\ \frac{1}{0} \cdot - 3 & \frac{1}{0} \cdot 9 \end{array}]$

Rearrange $\frac{1}{0} \cdot \frac{1}{3}$ .

$[\begin{array}{c} Undefined & \frac{1}{0} \cdot - 1 \\ \frac{1}{0} \cdot - 3 & \frac{1}{0} \cdot 9 \end{array}]$

Since the matrix is undefined, it cannot be solved.

$Undefined$

Undefined