因为∠A+∠B+∠C=180°
又因为,∠B+∠C=2∠A
所以
3∠A=180°
∠A=60°
∠B+∠C=120°
因为∠B:∠A=5:3=
∠B=60÷3×5=100°
∠C=120°-100°=20°