Let a^,b^ and c^ be three unit vectors inclined to each other at an angle θ, then the maximum value of θ is
2π3
5π3
π3
π2
(a^+b^+c^)2≥0⇒ 3+2(a^⋅b^+b→⋅c^+c→⋅a→)≥0⇒ 3+6cosθ≥0⇒ cosθ≥−12⇒ θ=2π3