Đáp án+Giải thích các bước giải:
a, $ABCD$ là hình thang, $AB//CD$
$⇒\widehat{A}+\widehat{D}=180^o$
$⇒\widehat{A}=180^o-\widehat{D}=180^o-60^o=120^o$
Vậy $\widehat{A}=120^o$
b, Ta có:
$\dfrac{\widehat{B}}{\widehat{C}}=\dfrac{4}{5}$
$⇒\dfrac{\widehat{B}}{4}=\dfrac{\widehat{C}}{5}=\dfrac{\widehat{B}+\widehat{C}}{4+5}=\dfrac{180^o(hthABCD)}{9}=20^o$
$\dfrac{\widehat{B}}{4}=20^o⇒\widehat{B}=20^o.4=80^o$
$\dfrac{\widehat{C}}{5}=20^o⇒\widehat{C}=20^o.5=100^o$
Vậy $\widehat{B}=80^o,\widehat{C}=100^o$
