It is known that the opposite sides of the internal angles a, B and C of triangle ABC are a, B and C respectively. If a & # 178; + B & # 178; - C & # 178; + AB = 0, then the size of angle c is?

It is known that the opposite sides of the internal angles a, B and C of triangle ABC are a, B and C respectively. If a & # 178; + B & # 178; - C & # 178; + AB = 0, then the size of angle c is?

From the question consciousness, A2 B2 = C2 AB, so COSC = A2 b2-c2 / 2Ab = c2-ab-c2 / 2Ab = - 1 / 2. So C is 120 degrees