Trong tam giác DIA, ta có

GócIDC+gócDIC+gócICD=180'

hayGócIDC+gócICD=180'-gócDIC=65'

=>GócD+gócC2=65′GócD+gócC2=65′=>gócD+gócC=130'

Tứ giác ABCD,ta có

GócA+gócB+GócC+gócD=360'

hay gócA+gócB+130'=360'

=> gócA+gócB = 230'

Ta có: gócA+gócB=230'(1)

gócA-gócB=50'(2)

Thế (2) vào (1), ta có

50'+gócB+gócB=230'

hay 50'+2B =230'

2B = 180'

gócB = 90'

Lại có gócA+gócB=230'

hay gócA+90' =230'

=> gócA = 140'

Vậy gócA=140', gócB=90'