Giải: Vì AB // CD

    => A + D =180^o 

    mà A = 3D => 3D + D = 180^o

                        =>  4D = 180^o

                        =>   D = 45^o   => A = 135^o

Ta có: AB // CD => B + C = 180^o

        mà B - C = 30^o  hay B = C + 30^o

=> C + 30^o + C = 180^o

=>  2C = 150^o  => C = 75^o  => B = 105^o

 đây bạn ơi

Theo đề bài ta có:

AB // CD

`->` $\widehat{A}$+$\widehat{D}$`=180^0`

$\widehat{3D}$ + $\widehat{D}$ `=180^0`

$\widehat{4D}$`=180^0`

$\widehat{D}$`=180^0 : 4=45^0`

Vậy $\widehat{A}$ `=3.45^0=135^0`

Lai có:

$\widehat{B}$+$\widehat{C}$`=180^0`

mà $\widehat{B}$ - $\widehat{C}$`=30^0` hay $\widehat{B}$ = $\widehat{C}$ `+30^0`

Vậy `30^0`+ $\widehat{C}$+ $\widehat{C}$`=180^0`

$\widehat{2C}$`=180^0-30^0=150^0`

$\widehat{C}$`=150^0 : 2=75^0`

`=>`$\widehat{B}$ `=75^0 +30^0=105^0`