Let be a solution of x2 + x + 1 = 0, and for some a and b in R, bmatrix 4 & a & b bmatrix bmatrix 1 & 16 & 13 \ -1 & -1 & 2 \ -2 & -14 & -8 bmatrix = bmatrix 0 & 0 & 0 bmatrix. If 4 4 + m a + n b = 3, then m + n is equal to
- 11
- 3
- 8
- 7
Answer
(1) 11
Solution
Multiplying the matrices and equating each element to 0:
4 - a - 2b = 0 … (i)
64 - a - 14b = 0 … (ii)
52 + 2a - 8b = 0 … (iii)
(ii) − (i): 60 - 12b = 0 b = 5, and then a = 4 - 10 = -6 (these also satisfy (iii)).
is a non-real root of x2 + x + 1 = 0 3 = 1, 2 + + 1 = 0
4 = , -6 = 1, 5 = 2
4 + m + n 2 = 3 4 2 + m + n = 3
4(-1 - ) + m + n = 3 (m - 7) + (n - 4) = 0
is non-real m = 7, n = 4
m + n = 11
Answer: (1)