Question

The minors of – 4 and 9 and the co-factors of – 4 and 9 in determinant $\left| \begin{matrix}

• 1 & - 2 & 3 \
• 4 & - 5 & - 6 \
• 7 & 8 & 9 \end{matrix} \right|$ are respectively.

• A. 42, 3; – 42, 3
• B. –42, –3 ; 42, –3
• C. 42, 3; – 42, – 3
• D. 42, 3; 42, 3

Answer 1

Answer: (B)

–42, –3 ; 42, –3

Answer 2

Minor of – 4 = $\left| \begin{matrix}

• 2 & 3 \ 8 & 9 \end{matrix} \right| = - 42$, 9 =$\left| \begin{matrix}
• 1 & - 2 \
• 4 & - 5 \end{matrix} \right| = - 3$

and cofactor of – 4 =$( - 1)^{2 + 1}( - 42) = 42$,

cofactor of 9= $( - 1)^{3 + 3}( - 3) = - 3$.