perhatikan gambar di samping.!besar

180=3x+50+2x-20
180=5x+30
5x=180-30
5x=150
x=30
ABD=3(30)+50
ABD=140

(^ = derajat)
3x + 50^ + 2x -20^ =180^
5x + 30^ = 180^
5x = 180^ - 30^
x = 150^ : 5
x = 30^

jadi, besar ABD = 3x + 50^
                             3(30^) +50^
                             90^ + 50^ + = 140^