如图，将A,B,C坐标代人抛物线方程，解a,b,c
E点坐标为(m, 2m+6)
F点坐标为(m, -m^2 - 2m+3)
EF = -m^2 - 4m-3
s = AEF面积+DEF面积 = 1/2 EF * 2 = -m^2 - 4m - 3 =1 -(m+2)^2
当m = -2时，s = 1 为最大值
