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连接CE.
∠B=180°-∠BEC-∠BCE,∠D=180°-∠DEC-∠DCE.
∠F=180°-∠FEC-∠FCE=180°-(∠BEC+1/2∠BED)-(∠BCE-1/2∠BCD)
=180°-∠BEC-∠BCE+1/2∠BCD-1/2∠BED
=∠B+1/2∠BCD-1/2∠BED
另有∠F=180°-∠FEC-∠FCE=180°-(∠DEC-1/2∠BED)-(∠DCE+1/2∠BCD)
=180°-∠DEC-∠DCE-1/2∠BCD+1/2∠BED
=∠D-1/2∠BCD+1/2∠BED
所以2∠F=(∠B+1/2∠BCD-1/2∠BED)+(∠D-1/2∠BCD+1/2∠BED)=∠B+∠D
∠F=(∠B+∠D)/2
1年前
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