解：(x+y)³-x³-y³+3xy =x³+y³+3x²y+3xy²-x³-y³+3xy =3x²y+3xy²+3xy =3xy(x+y+1)
(x+y)3-x3-y3+3xy =x³+3x²y+3xy²+y³-x³-y³+3xy=3x²y+3xy²+3xy=3xy(x+y+1)
