证明:
∠EHF=180-∠HEF-∠HFE=180-(∠AEF-∠AEH)-(∠AFE-∠AFH)
=180-(∠AEF+∠AFE)+1/2(∠AED+∠AFB)=∠A+1/2(∠BCD-∠A)
=1/2(∠A+∠BCD)=1/2(180)=90°
故EH⊥FH
证明:
∠EHF=180-∠HEF-∠HFE=180-(∠AEF-∠AEH)-(∠AFE-∠AFH)
=180-(∠AEF+∠AFE)+1/2(∠AED+∠AFB)=∠A+1/2(∠BCD-∠A)
=1/2(∠A+∠BCD)=1/2(180)=90°
故EH⊥FH