设∠A=5x ∠B=7x ∠C=2x ∠D=2x+80
∠A+∠B+∠C+∠D=360°
5x+7x+2x+2x+80=360°
16x=280°
x=17.5
∠A=87.5° ∠B=122.5° ∠C=35° ∠D=115°