JAMB 2014 · UTME · Q23

Matrices & determinants

If $\begin{vmatrix} -x & 12 \\ -1 & 4 \end{vmatrix} = -12$ , find $x$ .

Worked solution (try it first)

A $2 \times 2$ determinant is $ad - bc$ : $(-x)(4) - (12)(-1) = -4x + 12$ .
So $-4x + 12 = -12$ .
Take 12 from both sides: $-4x = -24$ .
Divide by $-4$ : $x = 6$ , option A.

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