(x+y+z)³
=X³+y³+z³+3x²y+3²z+3xy²+3y²z+3xz²+3yz²+6xyz
=X³+y³+z³+3xy(x+y+z)+3xz(x+y+z)+3yz(x+y+z)-3xyz
∵x+y+z=0
∴X³+y³+z³=3xyz