$ABCD$ là hình thang
$→AB//CD$
$→\widehat B+\widehat C=180^\circ$ (2 góc trng cùng phía)
$\dfrac{\widehat B}{\widehat C}=\dfrac{4}{5}$
$→\widehat B=\dfrac{4\widehat C}{5}$
Thay $\widehat B=\dfrac{4\widehat C}{5}$ vào $\widehat B+\widehat C=180^\circ$
$\dfrac{4\widehat C}{5}+\widehat C=180^\circ\\→\dfrac{4\widehat C}{5}+\dfrac{5\widehat C}{5}=180^\circ\\→\dfrac{9\widehat C}{5}=180^\circ\\→9\widehat C=900^\circ\\→\widehat C=100^\circ\\→\widehat B=\dfrac{4.100^\circ}{5}=80^\circ$
Vậy $\widehat B=80^\circ,\widehat C=100^\circ$
