(x-2分之1y-1)(x-2分之1y+1)-(x-2分之1y-1)的平方
=(x-1/2y-1)(x-1/2y+1-x+1/2y+1)
=2(x-1/2y-1)
=2x-y-2
=2×1.5-0.9-2
=0.1