Đáp án:
$\begin{array}{l}
a)Do:\widehat {{B_1}} = \widehat {{C_1}}\\
\Rightarrow AB//CD\left( {\text{do 2 góc so le trong bằng nhau}} \right)\\
\text{Vậy}\,AB//CD\\
b)Do:AB//CD\\
AD \bot CD\\
\Rightarrow AB \bot AD\\
\text{Vậy}\,AB \bot AD\\
c)AB//CD\\
\Rightarrow \left\{ {\widehat {{B_2}} = \widehat {{C_2}} = {{110}^0}\left( {so\,le\,trong} \right)} \right.\\
\widehat {{C_3}} + \widehat {{C_2}} = {180^0}\\
\Rightarrow \widehat {{C_3}} = {180^0} - {110^0} = {70^0}\\
\text{Vậy}\,\widehat {{C_2}} = {110^0};\widehat {{C_3}} = {70^0}
\end{array}$
