WAEC 2020 · Paper 1 · Q2

Find the inverse of $\begin{pmatrix} 3 & 5 \\ 1 & 2 \end{pmatrix}$ .

Worked solution (try it first)

The determinant is $ad - bc = 3 \times 2 - 5 \times 1 = 1$ .
For the inverse, swap the leading diagonal ( $3$ and $2$ ) and change the signs of the other two entries: $\begin{pmatrix} 2 & -5 \\ -1 & 3 \end{pmatrix}$ .
Divide by the determinant, 1, which changes nothing.
The inverse is $\begin{pmatrix} 2 & -5 \\ -1 & 3 \end{pmatrix}$ , option B.