Let A = bmatrix 2 & a & 0 \ 1 & 3 & 1 \ 0 & 5 & b bmatrix. If A3 = 4A2 - A - 21I, where I is the identity matrix of order 3 3, then 2a + 3b is equal to
- -10
- -12
- -13
- -9
Answer
(3) -13
Solution
Cayley–Hamilton: A satisfies its characteristic equation 3 - (tr A) 2 + k - |A| = 0
Given: A3 - 4A2 + A + 21I = O
Comparing the two cubics, tr A = 4 and |A| = -21
tr A = 2 + 3 + b = 4 b = -1
|A| = 2(3b - 5) - a(b - 0) = 6b - 10 - ab
-6 - 10 + a = -21 a = -5
2a + 3b = -10 - 3 = -13
Answer: (3)