Rút gọn : x^2 + y^2 + z^2 / ( y - z )^2 + ( z - x )^2 + ( x - y )^2

Rút gọn : x^2 + y^2 + z^2 / ( y - z )^2 + ( z - x )^2 + ( x - y )^2 biết rằng x + y + z = 0