2.·AA=AE AA=E=A”E=4A A‖A=4 19.若40则A=4 2.若A=0,则4=0 由1.知 A1=0A1-4 证毕 * 2. A A A E = * n n  = = = A A A E A E A * n  = A A A 由1.知 * A = 0 * n 1 A A −  = 证毕 A  0, 0 1 . 若 则 1 * n A A − = 若 A = 0, 则 1 0 n A − = 0 2