AF=FG=GC
=>AG=2/3AC
=>S ABG=2/3*S ABC
BE=ED=1/2AD
=>BE+ED=1/2AD+1/2AD=AD
=>BD=DA
=>D là trung điểm của AB
=>AD=1/2AB
=>BE=ED=1/4AB
AE=AD+DE=1/4AB+1/2AB=3/4AB
=>S AEG=3/4*S ABG=3/4*2/3*S ABC=1/2*S ABC
=>S BEGC=1/2*S ABC
AF=FG
=>AF=1/2*AG
=>S AEF=1/2*S AEG=1/2*1/2*S ABC=1/4*S ABC
AD=2DE
=>AD=2/3AE
=>S ADF=2/3*S AEF=2/3*1/4*S ABC=1/6*S ABC
=>s EDFG=S ABC(1/4-1/6)=1/12*S ABC
S BCGE-S DEFG=15cm2
=>S ABC(1/2-1/12)=15
=>SABC=15:5/12=36cm2
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