8．a,b,c为△ABC的三边3a^3+6a^2b-3a^2c-6abc=O,

3a^3+6a^2b-3a^2c-6abc=Oa^3+2a^2b-a^2c-2abc=0∴a(a^2+2ab-ac-2bc)=0∴a^2+2ab-ac-2bc=0∴a(a+2b)-c(a+2b)=0∴(a-c)(a+2b)=0∵a+2b>0∴a-c=0,即a=c∴△ABC是等腰三角形