设CF和DE交于点O
证明:
∵AE=DF AD=DC ∠EAD=∠FDC
∴△EAD≌△FDC
∴∠AED=∠DFC
又∠ADE+∠AED=90°
∴∠ADE+∠DFC=90°
∴∠FOD=90°
∴CF⊥DE