soluzione Scomporre       x6 - 5x4 + 4x2 = = x2 (x4 - 5x2 + 4) = = x2 (x2 - 1) (x2 - 4) = = x2 (x - 1) (x + 1) (x - 2) (x + 2) = = x2 (x - 2) (x - 1) (x + 1) (x + 2)