phan tinh ra thi o=2a^2+2b^2+2c^2-2ab-2ac-2bc

                      0=(a-b)^2+(a-c)^2+(b+c)^2

                      suy ra (a-b)^2>=0 (1)

                               (a-c)^2>=0 (2)

                               (b-c)^2>=0 (3)

tu 1 va 2 suy ra a=b (4)

tu1va 3 suy ra a=c (5)

tu 4 va 5 suy ra a=b=c (dpcm)