矩形ABCD的长为a,宽为b,ab=1.
AE=a/5,BE=4a/5,b=1/a.
1.计算F点在CD上的位置:
S△BEH=BE*BC/2-S△BCH
=(4a/5)*(1/a)/2-1/8
=11/40
h1=2*S△BEH/BE (h1为△BEH之BE边上的高)
=2*(11/40)/(4a/5)
=55/(80a)
S△CFH=CF*(b-h1)/2=CF*BC/2-S△BCH
CF*[1/a-55/(80a)]/2=CF*(1/a)/2-1/8
CF*25/(160a)=CF*(80/160a)-20/160
CF*(55/160a)=20/160
CF=4a/11
DF=DC-CF=7a/11
2.计算△ADG的面积:
S△ADG=S△ADE-S△AEG
=AE*AD/2-AE*h2/2 (h2为△AEG之AE边上的高)
=(a/5)*(1/a)/2-(a/5)*h2/2
=1/10-(a/10)*h2 ------------(1)
S△ADG=S△ADF-S△DFG
=DF*AD/2-DF*(AD-h2)/2
=(DF*h2)/2
=[(7a/11)*h2]/2
=(7a/22)*h2 ------------(2)
(2)代入(1)
(7a/22)*h2=1/10-(a/10)*h2
(70a/220)*h2=22/220-(22a/220)*h2
h2=22/(92a)
S△ADG=(7a/22)*h2
=7/92
答:△ADG的面积是7/92.