Ta có:
$\widehat{B}$ $+$ $\widehat{C}$ $=$ $180^o$ (hai góc ở vị trí trong cùng phía)
Mà $\widehat{B}$ $=$ $2\widehat{C}$ nên ta có:
$2\widehat{C}$ $+$ $\widehat{C}$ $=$ $180^o$
$\Leftrightarrow$ $3\widehat{C}$ $=$ $180^o$
$\Leftrightarrow$ $\widehat{C}$ $=$ $60^o$
mà $\widehat{B}$ $=$ $2\widehat{C}$ nên $\widehat{B}$ $=$ $60^o$ $. 2 = $ $120^o$
Ta có: $\widehat{A}$ $+$ $\widehat{D}$ $=$ $180^o$ (hai góc ở vị trí trong cùng phía)
Mà $\widehat{A}$ $-$ $\widehat{D}$ $=$ $20^o$
$\Rightarrow$ $(\widehat{A}$ $+$ $\widehat{D})$ $+$ $(\widehat{A}$ $-$ $\widehat{D})$ $=$ $200^o$
$2\widehat{A}$ $=$ $200^o$ $\Rightarrow$ $\widehat{A}$ $=$ $100^o$
Do $\widehat{A}$ $-$ $\widehat{D}$ $=$ $20^o$
$\Rightarrow$ $\widehat{D}$ $=$ $80^o$
Vậy $\widehat{A}$ $=$ $100^o$; $2\widehat{B}$ $=$ $120^o$; $\widehat{C}$ $=$ $60^o$; $\widehat{D}$ $=$ $80^o$
