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