If a, b and c are unit vectors such that a x (b x c) = ½b, then angle between a and c is?

1) π/6

2) π/4

3) π/2

4) π/3

Answer: (4) π/3

Solution: Given,

a x (b x c) = ½b

(a.c)b – (a.b)c = ½ b

If we compare both the sides,

a.c = ½

|a| |c| cos θ = ½

Cos θ = ½

Therefore,

θ = π/3

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