Let 0 and A = bmatrix & & 3 \ & & \ - & & 2 bmatrix. If B = bmatrix 3 & -9 & 3 \ - & 7 & -2 \ -2 & 5 & -2 bmatrix is the matrix of cofactors of the elements of A, then (AB) is equal to :
- 64
- 343
- 125
- 216
Answer
(4) 216
Solution
C11 = 2 2 - = 3 , 0
2 - = 3
C13 = 2 + = 3 + = 3
= 2, = 1 (check: C12 = -(2 2 + 2) = -9 ✓)
|A| = vmatrix 1 & 2 & 3 \ 2 & 2 & 1 \ -1 & 2 & 4 vmatrix = 1(6) - 2(9) + 3(6) = 6
BT = adj A |B| = | adj A| = |A|2 = 36
(AB) = |A| |B| = 6 36 = 216
Answer: (4)