顶点A,底边BC(左B,右C),BD交AC于D,CE交AB于E,
BD与CD交于F,
S△ABD+S△DBC=S△ABC=1,
S△DBC=2S△ABD,【等高】
3S△ABD=1
S△ABD=1/3;
同理S△CAE=2/5;
连接AF,
设S△AFD=S1,S△DFC=2S△AFD=2S1;
S△AFE=2S2,S△EFB=3S2;
S1+2S2+3S2=1/3
2S2+S1+2S1=2/5
解方程组得S1=4/39,S2=3/65;
阴影面积=1-3S1-5S2=1-3*4/39-5*3/65=6/13.
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