Lặn lâu rồi h mình lên tý:>

Vì a//b nên ta có
$\displaystyle \begin{cases}
\widehat{C_{2}} & +\ \widehat{D_{2}} =\ 180^{0}\\
\widehat{C_{2}} & -\ \widehat{D_{2}} =\ 30^{0}
\end{cases}$
$\displaystyle \begin{cases}
\widehat{C_{2}} & =\ 105^{0}\\
\widehat{D_{2}} & =\ 75\ ^{0} \ 
\end{cases}$
Có $\displaystyle  \begin{array}{{>{\displaystyle}l}}
\widehat{C_{1}} =\ 180^{0} -\widehat{C_{2}} =180^{0} -\ 105^{0} =\ 75\ ^{0} \ ( Vì\ 2\ góc\ kề\ bù)\\
\ \widehat{D_{1}} \ =\ \ 180^{0} -\widehat{DD_{2}} \ =180^{0} -\ 75^{0} =\ 105^{0} \ ( Vì\ 2\ góc\ kề\ bù)
\end{array}$