Bước 1 : Do `a//b=>` $\widehat{A3}$`=`$\widehat{B3}$`=80^o` (2 góc so le trong )
Bước 2 : Do `a//b=>`$\widehat{B3}$=$\widehat{A1}$`=80^o` (2 góc so le ngoài )
mà $\widehat{A3}$=$\widehat{A1}$
`=>`$\widehat{A3}$=`80^o`
Bước 3 : Ta có : $\widehat{B3}$=$\widehat{B1}$`=80^o` ( 2 góc đối đỉnh )
mà $\widehat{A3}$=$\widehat{B1}$ ( 2 góc so le trong )
`=>`$\widehat{A3}$`=80^o`
Bước 4: Ta có : $\widehat{B3}$+$\widehat{B2}$`=180^o` ( 2 góc kề bù )
`=>` $\widehat{B2}$`=100^o`
mà $\widehat{B2}$+$\widehat{A3}$`=180^o`( 2 góc trong cùng phía )
`=>`$\widehat{A3}$=`80^o`
Bước 5 : Ta có: $\widehat{B3}$=$\widehat{B1}$
`=>`$\widehat{B1}$+$\widehat{A4}$=`180^o`
`=>`$\widehat{A4}$=`100^o`
`=>$\widehat{A3}$=`80^o`
Bước 6 : Ta có : $\widehat{A2}$=$\widehat{B2}$=`100^o`
`=>`$\widehat{A3}$=`80^o`
$\text{Xin hay nhất }$