a) $\widehat{D}+\widehat{A}=180^o$

$↔ \widehat{A}=180^o-\widehat{D}=180^o-70^o=110^o$

$\widehat{B}+\widehat{C}=180^o$

$↔ \widehat{C}=180^o-\widehat{B}=180^o-50^o=130^o$

b) $\widehat{D}+\widehat{A}=180^o$

$↔ \widehat{A}=180^o-\widehat{D}=180^o-120^o=60^o$

$\widehat{B}=180^o-40^o=140^o$

$\widehat{C}=40^o$

a) $ABCD$ là hình thang

$→\widehat{A}+\widehat{D}=180^\circ$ (2 góc kề cạnh bên)

$→\widehat{B}+\widehat{C}=180^\circ$ (2 góc kề cạnh bên)

mà $\widehat{D}=70^\circ,\widehat{B}=50^\circ$

$→\widehat{A}=180^\circ-70^\circ=110^\circ$

$→\widehat{C}=180^\circ-50^\circ=130^\circ$

b) Giả sử: $\widehat{B_1},\widehat{D_1}$ là 2 góc ngoài tại đỉnh $B,D$

$→\widehat{B_1}+\widehat{ABC}=180^\circ$ (kề bù)

$→\widehat{D_1}+\widehat{ADC}=180^\circ$ (kề bù)

mà $\widehat{B_1}=40^\circ,\widehat{D_1}=120^\circ$

$→\widehat{ABC}=180^\circ-40^\circ=140^\circ$

$→\widehat{ADC}=180^\circ-120^\circ=60^\circ$

$ABCD$ là hình thang

$→\widehat{A}+\widehat{ADC}=180^\circ$ (2 góc kề cạnh bên)

$→\widehat{C}+\widehat{ABC}=180^\circ$ (2 góc kề cạnh bên)

mà $\widehat{ADC}=60^\circ,\widehat{ABC}=140^\circ$

$→\widehat{A}=180^\circ-60^\circ=120^\circ$

$→\widehat{C}=180^\circ-140^\circ=40^\circ$