Given the trilateral length of △ ABC, a, B, C satisfies a & # 178; = B & # 178; + C & # 178; + BC, find ∠ a

According to the cosine theorem
cosA=(b^2+c^2-a^2)/(2bc)
It is known that a & # 178; = B & # 178; + C & # 178; + BC
So cosa = (- BC) / (2BC) = - 0.5
In the triangle, cosa

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