(後只人季 (1)A±B,kA也是上三角矩阵 kan 4= 0 ka (2)AB也是上三角,并且 1111 AB 22-22 0 nn11 22 * 0 nn ka ka kA ka       =       11 11 22 22 * 0 nn nn a b a b AB a b       =      