Let the angle , 0 < < 2 between two unit vectors a and b be -1 ( 659 ). If the vector c = 3 a + 6 b + 9( a b), then the value of 9( c a) - 3( c b) is
(2) 29
= 659 = 1 - 6581 = 49 ( acute)
a b = 49, and ( a b) a = ( a b) b = 0
c a = 3 + 6 ( 49 ) = 3 + 83 = 173
c b = 3 ( 49 ) + 6 = 43 + 6 = 223
9( c a) - 3( c b) = 51 - 22 = 29
Answer: (2)
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