∵∠A+∠B+∠C=180°又因为∠A+∠B=2∠C
∴2∠C+∠C=180°
3∠C=180°
∠C=60°
∴∠A+∠B=2∠C=120°
∵∠A－∠B=80°又因为∠A+∠B=120°
∴∠A－∠B+∠A+∠B=80°+120°
2∠A=200°
∠A=100°
∠B=120°-∠A=20°
∴∠A=100°
∠B=20°
∠C=60°