a^ 2+2b^ 2+c^ 2-2ab-6b-2c+7=0
a^ 2-2ab+b^ 2+b^2-6b+9+c^ 2-2c+1=0
(a-b)^2+(b-3)^2+(c-1)^2=0
所以a=b,b=3,c=1