Let A= bmatrix & 1 & 2\ 2 & 3 & 0\ 0 & 4 & 5 bmatrix and B= bmatrix1 & 0 & 0\ 0 & -5 & 0\ 0 & 4 & -2 bmatrix+adj(A). If (B)=66, then (adj(A)) equals :
- 289
- 361
- 441
- 529
Answer
(3) 441
Solution
Cofactors of A: C11=15, C12=-10, C13=8; C21=3, C22=5 , C23=-4 ; C31=-6, C32=4, C33=3 -2
adj(A)= bmatrix15 & 3 & -6\ -10 & 5 & 4\ 8 & -4 & 3 -2 bmatrix
B= bmatrix16 & 3 & -6\ -10 & 0 & 4\ 8 & 0 & -2 bmatrix
Expanding along the second column (only one non-zero entry):
(B)=-3 vmatrix-10 & 4\ 8 & -2 vmatrix=-3(-10 +20-32)=30 +36
30 +36=66 =1
(A)= (15)-1(10)+2(8)=15 +6=21
(adj(A))=( A)3-1=212=441
Answer: (3)