過點A做直線AC//GN,如圖所示,連線CD、BE,作BK⊥AC於K,CH⊥AB於H,
設直線DE和BC之間的距離為h,則:
S△ABC:S△ADC =1/2AB*CH:1/2AD*CH = AB:AD
S△ABC:S△AEB =1/2AC*BK: 1/2AE*BK = AC:AE
S△DBC=1/2BC*h=S△EBC(同底等高)
∵S△ADC=S△ABC-S△DBC
S△AEB=S△ABC-S△EBC=S△ABC-S△DBC
∴S△ADC=S△AEB
∴S△ABC:S△ADC=S△ABC:S△AEB
∴AB:AD=AC:AE
即:(AD+BD):AD=(AE+EC):AE
即:1+BD:AD=1+EC:AE
∴BD:AD=EC:AE
即:AD:BD=AE:EC
過點A做直線AC//GN,如圖所示,連線CD、BE,作BK⊥AC於K,CH⊥AB於H,
設直線DE和BC之間的距離為h,則:
S△ABC:S△ADC =1/2AB*CH:1/2AD*CH = AB:AD
S△ABC:S△AEB =1/2AC*BK: 1/2AE*BK = AC:AE
S△DBC=1/2BC*h=S△EBC(同底等高)
∵S△ADC=S△ABC-S△DBC
S△AEB=S△ABC-S△EBC=S△ABC-S△DBC
∴S△ADC=S△AEB
∴S△ABC:S△ADC=S△ABC:S△AEB
∴AB:AD=AC:AE
即:(AD+BD):AD=(AE+EC):AE
即:1+BD:AD=1+EC:AE
∴BD:AD=EC:AE
即:AD:BD=AE:EC