For three unit vectors a, b, c satisfying | a - b|2 + | b - c|2 + | c - a|2 = 9 and |2 a + k b + k c| = 3, the positive value of k is :
- 4
- 5
- 6
- 3
Answer
(2) 5
Solution
| a - b|2 + | b - c|2 + | c - a|2 = 6 - 2( a b + b c + c a) = 9
a b + b c + c a = - 32
| a + b + c|2 = 3 + 2 (- 32 ) = 0
a + b + c = 0 b + c = - a
|2 a + k( b + c)| = |(2 - k) a| = |2 - k| = 3
k = 5 or k = -1
Positive value: k = 5
Answer: (2)