Vì $\widehat{A_1}+\widehat{A_2}=180^o$
mà $\widehat{A_1}=\dfrac{5}{7}\widehat{A_2}$
$→\dfrac{5}{7}\widehat{A_2}+\widehat{A_2}=180^o$
$→\dfrac{12}{7}\widehat{A_2}=180^o$
$→\widehat{A_2}=105^o$
Vì $\widehat{B_2}>\widehat{B_1}$ khoảng $30^o$
$→\widehat{B_2}-\widehat{B_1}=30^o$
$→\widehat{B_1}=\widehat{B_2}-30^o$
mà $\widehat{B_1}+\widehat{B_2}=180^o$
$→\widehat{B_2}-30^o+\widehat{B_2}=180^o$
$→2.\widehat{B_2}=210^o$
$→\widehat{B_2}=105^o$
Từ hai điều trên $→\widehat{A_2}=\widehat{B_2}$
mà 2 góc ở vị trí so le trong
$→a//b$ mà $c⊥b$
$→c⊥a$ (từ ⊥ đến //)
