例 103-1 11 A B 2102 03 9-2-1 C=AB= 9911 5 2021/2/202021/2/20 5 例       − − = =             − =       − = 9 9 11 9 2 1 1 3 4 2 0 1 1 1 3 4 1 0 , 2 1 0 2 1 0 3 1 C AB A B