Kẻ một đường t qua C và Ct// Ax mà Ax//Dy suy ra Ct//Ax//Dy.
Ta có: Ct//Ax suy ra $\widehat{CAx}=\widehat{ACt}=50^{\circ}$ (hai góc so le trong)
Ta có: $\widehat{ACt}+\widehat{DCt}=\widehat{DCA}$
=>$ 50^{\circ}+\widehat{DCt}=110^{\circ}$
=> $\widehat{DCt}=110^{\circ}-50^{\circ}=60^{\circ}$
Vì Dy//Ct => $\widehat{DCt}=\widehat{CDy}=60^{\circ}$ (hai góc so le trong)
Vậy $\widehat{CDy}=60^{\circ}$