$\widehat{{{C}_{1}}}-\widehat{{{C}_{2}}}={{40}^{0}}\Rightarrow \widehat{{{C}_{1}}}={{40}^{0}}+\widehat{{{C}_{2}}}$.Mặt khác ta có:

$\widehat{{{C}_{1}}}+\widehat{{{C}_{2}}}={{180}^{0}}$

$\Rightarrow {{40}^{0}}+\widehat{{{C}_{2}}}+\widehat{{{C}_{2}}}={{180}^{0}}.$

$\Rightarrow \widehat{{{C}_{2}}}={{70}^{0}}.$

 a//b nên ta có : $\widehat{{{D}_{1}}}=\widehat{{{C}_{2}}}={{70}^{0}}$(hai góc so le trong)