原式=[x+(2y-1)][x-(2y-1)]-[(x-2y)-1]2
=x2-(2y-1)2-[(x-2y)2-2(x-2y)+1]
=x2-(4y2-4y+1)-(x2-4xy+4y2-2x+4y+1)
=x2-4y2+4y-1-x2+4xy-4y2+2x-4y-1
=4xy-8y2+2x-2