AE=DF=x
DE=2-x
S(DEF)=(2x-x^2)/2
又由(2)
S(DEOF)=S(ABCD)/4=1
所以
y=S(EOF)=1-(2x-x^2)/2=(x^2-2x+2)/2
即y=f(x)=0.5x^2-x+1(0