cho a^2+b^2+c^2-ab-ac-bc=0 Chứng minh rằng:a=b=c

 `a^2+b^2+c^2-ab-ac-bc=0`

`<=> 2a^2+2b^2+2c^2-2ab-2ac-2bc=0`

`<=> (a^2-2ab+b^2)+(b^2-2bc+c^2)+(a^2-2ac+c^2)=0`

`<=> (a-b)^2+(b-c)^2+(a-c)^2=0`

`<=> a – b = b – c = a – c = 0`

`<=> a = b = c `