mA+mB=180°
mB+mC=180°
unghiurile alaturate sumt suplementare
2×mC+2×mb+18=180°
mB=2×mC=>
=>2×mC+2×(2×mC)=180°
2×mC+4×mC=180°
6mC=180°
mC=180/6
mC=30°
mB+mA=180°
mB+2×mB+18=180°
3mB=180-18
3mB=162°
mB=162/3
mB=54°
mB+mA=180°
54+mA=180
mA=180-54
mA=126°
mA+mB+mC+mD=360°
126+54+30+mD=360
210+mD=360°
mD=360-210
mD=150°