原式即(a^2-ab+b^2/4)+(3b^2/4-6b+12)+(c^2-6c+9)=0
即(a-b/2)^2+3/4(b-4)^2+(c-3)^2=0
所以a-b/2=0,b-4=0,c-3=0
得a=8,b=4,c=3