Vì ABCD là tứ giác nội tiếp nên \(\widehat{A} + \widehat{C} = 180^o\) và \(\widehat{B} + \widehat{D} = 180^o\).
a) \(\widehat{A} = 60^o,\ \widehat{B} = 80^o\)
\(\widehat{C} = 180^o - \widehat{A} = 180^o - 60^o = 120^o\)
\(\widehat{D} = 180^o - \widehat{B} = 180^o - 80^o = 100^o\)
b) \(\widehat{B} = 70^o,\ \widehat{C} = 90^o\)
\(\widehat{A} = 180^o - \widehat{C} = 180^o - 90^o = 90^o\)
\(\widehat{D} = 180^o - \widehat{B} = 180^o - 70^o = 110^o\)
c) \(\widehat{C} = 100^o,\ \widehat{D} = 60^o\)
\(\widehat{A} = 180^o - \widehat{C} = 180^o - 100^o = 80^o\)
\(\widehat{B} = 180^o - \widehat{D} = 180^o - 60^o = 120^o\)
d) \(\widehat{D} = 110^o,\ \widehat{A} = 80^o\)
\(\widehat{B} = 180^o - \widehat{D} = 180^o - 110^o = 70^o\)
\(\widehat{C} = 180^o - \widehat{A} = 180^o - 80^o = 100^o\)
