Given that the opposite sides of △ ABC's internal angles a, B and C are a, B, C, acosb + bcosa = csin (a-b), and A2 + B2 − 3AB = C2, the size of angle a is obtained

According to the sine theorem, ∵ acosb + bcosa = csin (a-b), ∵ sinacosb + sinbcosa = sincsin (a-b), ∵ sin (a + b) = sincsin (a-b), ∵ a + B + C = π∵ sin (a + b) = sinc ∵ sin (a-b) = 1,

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