Đáp án:
Theo `GT : hat{A} = hat{D} + 40^o`
`↔ hat{A} = hat{D} = 360^o : 2 - 40^o = 140^o`
`↔ hat{A} = 140^o : 2 = 70^o`
`↔ hat{D} = 70^o + 40^o = 110^o`
Theo `GT : hat{B} = 2hat{C}`
`↔ 2hat{C} = hat{B} = 360^o - hat{A} - (hat{A} = hat{D} + 40^o) = 360^o - 70^o - 140^o = 180^o`
`↔ hat{B} = 180^o : 3 = 60^o`
`↔ hat{C} = 60^o . 2 = 120^o`
Vậy `hat{A} = 70^o, hat{D} = 110^o, hat{B} = 60^o, hat{C} = 120^o`