Which one of the following matrices can be obtained by performing elementary row transformations on the 3 3 identity matrix ?
- bmatrix 1 & 1 & 1 \ 1 & 1 & 1 \ 1 & 1 & 1 bmatrix
- bmatrix 1 & 1 & 1 \ 2 & 3 & 4 \ 1 & 2 & 1 bmatrix
- bmatrix 1 & 1 & 1 \ 2 & 3 & 4 \ 2 & 5 & 8 bmatrix
- bmatrix 1 & 1 & 1 \ -1 & 1 & 2 \ 0 & 2 & 3 bmatrix
Answer
(2) bmatrix 1 & 1 & 1 \ 2 & 3 & 4 \ 1 & 2 & 1 bmatrix
Solution
Elementary row operations on I give a product of elementary matrices, which is always invertible; conversely every invertible matrix can be obtained. So we need 0.
(A): all rows equal, = 0.
(B): = 1(3 - 8) - 1(2 - 4) + 1(4 - 3) = -5 + 2 + 1 = -2 0
(C): = 1(24 - 20) - 1(16 - 8) + 1(10 - 6) = 4 - 8 + 4 = 0
(D): = 1(3 - 4) - 1(-3 - 0) + 1(-2 - 0) = -1 + 3 - 2 = 0
Answer: (B)