∠A+∠B+∠C=180°
∠B+∠C=2∠A代入
2∠A+∠A=180°
3∠A=180°
∠A=60°
∠B+∠C=120° (1)
∠B-∠C=40° (2)
(1)+(2)
2∠B=160°
∠B=80°