英文三角形數學題(15分)

AECF is a //gram. (given)
AF // CE (opp. sides of //gram AECF)
AFE = CEF (alt. s, AF // CE)
But AFE = 180o - AFD (adj. s on a st. line)
and CEF = 180o - CEB (adj. s on a st. line)
180o - AFD = 180o - CEB
AFD = AEB

AFD = AEB (proved)
DF = BE (given)
AF = CE (corr. sides of //gram AECF)
DAFD D CEB (SAS)

ADF = CBE (corr. s, DAFD D CEB)
AD // CB (alt. s equal)
AD = CB (corr. sides, DAFD D CEB)
Hence, ABCD is a // gram (opp. sides // and equal)
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