Cho x, y, z # 0. Chứng minh:(x^3/x^2 + xy + y^2) + (y^3/y^2 + yz + z^2) + (z^3/z^2 + zx + x^2) = (x^3/x^2 + xz + z^2)(y^3/y^2 + yx + x^2)(z^3/z^2 + zy + y^2)