Solve {ccc}{1}&{1}&{2}{0}&{-2}&{1}{0}&{1}&{2}

$$\left( \begin{array} { c c c } { 1 } & { 1 } & { 2 } \\ { 0 } & { - 2 } & { 1 } \\ { 0 } & { 1 } & { 2 } \end{array} \right)$$

Calculate Determinant

$$-5$$

Steps Using Sarrus' Rule

Find the determinant of the matrix using the method of diagonals.

$$det(\left(\begin{matrix}1&1&2\\0&-2&1\\0&1&2\end{matrix}\right))$$

Extend the original matrix by repeating the first two columns as the fourth and fifth columns.

$$\left(\begin{matrix}1&1&2&1&1\\0&-2&1&0&-2\\0&1&2&0&1\end{matrix}\right)$$

Starting at the upper left entry, multiply down along the diagonals, and add the resulting products.

$$-2\times 2=-4$$

Starting at the lower left entry, multiply up along the diagonals, and add the resulting products.

$$1=1$$

Subtract the sum of the upward diagonal products from the sum of the downward diagonal products.

$$-4-1$$

Subtract 1 from -4.

$$-5$$

Steps Using Expansion by Minors

Find the determinant of the matrix using the method of expansion by minors (also known as expansion by cofactors).

$$det(\left(\begin{matrix}1&1&2\\0&-2&1\\0&1&2\end{matrix}\right))$$

To expand by minors, multiply each element of the first row by its minor, which is the determinant of the 2\times 2 matrix created by deleting the row and column containing that element, then multiply by the element's position sign.

$$det(\left(\begin{matrix}-2&1\\1&2\end{matrix}\right))-det(\left(\begin{matrix}0&1\\0&2\end{matrix}\right))+2det(\left(\begin{matrix}0&-2\\0&1\end{matrix}\right))$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the determinant is ad-bc.

$$-2\times 2-1$$

Simplify.

$$-5$$

Evaluate

$$\left(\begin{matrix}1&1&2\\0&-2&1\\0&1&2\end{matrix}\right)$$