Let a = 2 i - 3 j + k, b = 3 i + 2 j + 5 k and a vector c be such that ( a - c) b = -18 i - 3 j + 12 k and a c = 3. If b c = d, then | a d| is equal to :
- 15
- 18
- 12
- 9
Answer
(1) 15
Solution
( a - c) b = a b - c b = a b + b c
a b = vmatrix i & j & k \ 2 & -3 & 1 \ 3 & 2 & 5 vmatrix = -17 i - 7 j + 13 k
d = b c = (-18 i - 3 j + 12 k) - (-17 i - 7 j + 13 k)
= - i + 4 j - k
(Check: b d = -3 + 8 - 5 = 0, as it must be.)
a d = 2(-1) + (-3)(4) + 1(-1) = -15
| a d| = 15
Answer: (1)