`#laviken#`
Vì `AB`//`CD` $\Rightarrow$ `ABCD` là hình thang
$\Rightarrow$ $\widehat{B}$ `+` $\widehat{C}$`=180^o`
mà $\widehat{B}$`-`$\widehat{C}$`=50^o`
$\Rightarrow$ $\widehat{B}$`=180^o +50^o/2=115^o`
$\Rightarrow$ $\widehat{C}$`=180^o-115^o=65^o`
Vì `AB`//`CD` $\Rightarrow$ $\widehat{A}$ `+` $\widehat{D}$`=180^o`
mà $\widehat{A}$`=1/3.`$\widehat{D}$
$\Rightarrow$ `1/3.`$\widehat{D}$`+`$\widehat{D}$`=180^o`
$\Rightarrow$ `4/3.`$\widehat{D}$`=180^o` $\Rightarrow$ $\widehat{D}$`=135^o`
$\Rightarrow$ $\widehat{A}$`=1/3.135^o=45^o`
Vậy $\widehat{A}$`=45^o`; $\widehat{B}$`=115^o`;$\widehat{C}$`=65^o`;$\widehat{D}$`=135^o`
