Let a = 2 i - j + 3 k, b = 3 i - 5 j + k and c be a vector such that a c = c b and ( a + c) ( b + c) = 168. Then the maximum value of | c|2 is :
- 77
- 154
- 308
- 462
Answer
(3) 308
Solution
a c = c b a c + b c = 0
( a + b) c = 0
c = t( a + b) = t(5 i - 6 j + 4 k)
a b = 6 + 5 + 3 = 14
a c = t(10 + 6 + 12) = 28t, b c = t(15 + 30 + 4) = 49t
| c|2 = t2(25 + 36 + 16) = 77t2
( a + c) ( b + c) = a b + a c + c b + | c|2 = 168
14 + 77t + 77t2 = 168
t2 + t - 2 = 0 t = 1 or t = -2
| c|2 = 77 or 77 4 = 308
maximum value of | c|2 = 308
Answer: (3)