△相似,
令FC与DG交点为P,正方形DEFG边长为m:PD/FE=CD/CEPD/m=15/(15+m)PD=15m/(15+m)PA = 15-PD = 15-15m/(15+m) = (225+15m-15m)/(15+m) = 225/(15+m)S△ACF= S△APF+S△APC = 1/2*AP*FG + 1/2*AP*AB = 1/2*AP*(FG+AB)= 1/2*225/(15+m)*(m+15)= 225/2= 112.5