The set of all , for which the vectors a = t i + 6 j - 3 k and b = t i - 2 j - 2 t k are inclined at an obtuse angle for all t R, is
- [0, 1)
- (- 43, 0 ]
- (-2, 0]
- (- 43, 1 )
Answer
(2) (- 43, 0 ]
Solution
For an obtuse angle, a b < 0 for all t:
a b = t2 - 12 + 6 t < 0 t R
= 0: -12 < 0 is always true, so = 0 is allowed
0: the quadratic is negative for all t only if < 0 and D < 0
D = 36 2 + 48 = 12 (3 + 4) < 0 - 43 < < 0
( a, b antiparallel would need = -3, outside this set)
(- 43, 0 ]
Answer: (2)