a,$\widehat{DCx}$=$\widehat{ACm}$=$60^o$ ( đối đỉnh )
b,$\widehat{mCx}$+$\widehat{ACm}$=$180^0$
⇒$\widehat{mCx}$=$180^0$-$60^0$=$120^0$
$\widehat{mCx}$=$\widehat{mDy}$ ( đồng vị )
⇒ Ax//By
c,$\left \{ {{BA\bot Ax} \atop {Ax//By}} \right.$ ⇒By$\bot$AB