Let A, B and C be three 2 × 2 matrices with real entries such that B = (I + A)-1 and A + C = I. If BC = bmatrix 1 & -5 \ -1 & 2 bmatrix and CB bmatrix x1 \ x2 bmatrix = bmatrix 12 \ -6 bmatrix, then x1 + x2 is
- 4
- 2
- 0
- −2
Answer
(3) 0
Solution
C = I − A, B = (I + A)-1
(I - A)(I + A) = I - A2 = (I + A)(I - A)
Pre- and post-multiplying by (I + A)-1:
(I + A)-1(I - A) = (I - A)(I + A)-1
BC = CB
bmatrix 1 & -5 \ -1 & 2 bmatrix bmatrix x1 \ x2 bmatrix = bmatrix 12 \ -6 bmatrix
x1 - 5x2 = 12 and -x1 + 2x2 = -6
Adding: -3x2 = 6 x2 = -2, x1 = 2
x1 + x2 = 0
Answer: (3)