Chứng minh răng: nếu: (x - y)^2 + (y - z)^2 + (z - x)^2 = (y + z - 2x)^2 + (z + x - 2y)^2 + (x + y - 2z)^2 thì x = y = z

CMR: Nếu: (x-y)^2+(y-z)^2+(z-x)^2= (y+z-2x)^2+(z+x-2y)^2+(x+y-2z)^2 thì x=y=z