Cho x y z thoả mãn x + y + z = 2 ; x^2 + y^2 + z^2 = 18 ; xyz = -1.Tính (1/xy + z - 1 ) + (1/yz + x - 1) + (1/xz + y - 1)

cho x y z  thoả mãn x + y + z = 2 ; x^2 + y^2 + z^2 = 18 ; xyz = -1 tính (1/xy + z - 1 ) + (1/yz + x - 1) + (1/xz + y - 1)