JAMB 1994 · UME · Q25

The determinant of the matrix $\begin{pmatrix} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 2 & 0 & -1 \end{pmatrix}$ is

Worked solution (try it first)

Expand along the first row, with signs $+ \; - \; +$ .
The first term is $1 \times (5 \times (-1) - 6 \times 0) = -5$ .
The second term is $-2 \times (4 \times (-1) - 6 \times 2) = -2 \times (-16)$
$= 32$ .
The third term is $3 \times (4 \times 0 - 5 \times 2) = 3 \times (-10)$
$= -30$ .
Add them: $-5 + 32 - 30 = -3$ , option C.