perhatikan gambar disamping ini.Besar < QRS =.....

Karena berpelurus
3x + (5x+20) =180
3x+5x+20 = 180
8x+20 = 180
8x = 180 -20
8x = 160
X = 160/8 = 20

< QRS = 5x + 20
= 5 (20) + 20 = 100 + 20 = 120 derajat