a+b=-c
(a+b)^3=-c^3
a^3+b^3+3a^2b+3ab^2=-c^3
a^3+b^3+c^3+3a^2b+3ab^2=0
3a^2b+3ab^2=0
ab(a+b)=0
得ab=0 或a+b=0
(1)a+b=0 则c=0 结论得证
(2)ab=0 可得a=0 b不等于0 则b+c=0 b=-c 结论得证
或a不等于0,b等于0则a+c=0 a=-c 结论得证.