(1+y)的平方-(y+1)(y-1)+2(1-y)的平方
=y²＋2y＋1-y²＋1＋2y²-4y＋2
=2y²-2y＋4
=2×（2分之1）²-2×2分之1＋4
=2分之1＋3
=3又2分之1