（x+y)³+2xy-2x²y-2xy²-1=(x+y)³-1 -2xy(x+y-1)=(x+y-1)[(x+y)²+(x+y)+1]-2xy(x+y-1)=(x+y-1)[(x+y)²+(x+y)+1-2xy]=(x+y-1)(x²+y²+x+y+1)