a. Ta có :Dx//CE
⇒$\mathop{C}\limits^{\displaystyle\frown}$=$\mathop{xDC}\limits^{\displaystyle\frown}$(2 góc so le trong )
⇒$\mathop{xDC}\limits^{\displaystyle\frown}$=45°
b. Vì tia Dx nằm giữa hai tia DC và DB
⇒$\mathop{CDx}\limits^{\displaystyle\frown}$+$\mathop{CDB}\limits^{\displaystyle\frown}$=115°
⇒$\mathop{xDB}\limits^{\displaystyle\frown}$=115°-45°=70°
Vì BA//Dx
⇒$\mathop{xDB}\limits^{\displaystyle\frown}$=$\mathop{C}\limits^{\displaystyle\frown}$(2 góc so le trong)
⇒$\mathop{B}\limits^{\displaystyle\frown}$=70°
