a²-2ab+2b²-2bc+2c²-6c+9=0(a²-2ab+b²)+(b²-2bc+c²)+(c²-6c+9)=0(a-b)²+(b-c)²+(c-3)²=0平分具有非负性即( )²≥0故(a-b)²=0,(b-c)²=0,(c-3)²=0a=b,b=c,c=32a+b-3c=0
化成(a-b)^2+(b-c)^2+(c-3)^2=0得a=b,b=c,c=32a+b-3c=0