If three unit vectors 𝐚, 𝐛 and 𝐜 satisfy 𝐚+𝐛+𝐜=0, then find the angle between 𝐚 and 𝐛

If three unit vectors 𝐚, 𝐛 and 𝐜 satisfy 𝐚+𝐛+𝐜=0, then find the angle between 𝐚 and 𝐛
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How get 2pi /3 plzz explain