可得到: 21 21 22 0/~女 =/:1E74≤|0a 22 2m m2可得到: = = ( ) m m ( ) m ( ) m ( ) ( ) m ( ) a a a a a a E P ;E A 1 1 2 1 2 1 2 2 1 1 1 1 2 1 1 1 0 0 1 0 0 1 1 1 2 1 2 2 1 2 1 1 2 1 2 1 1 1 a a a a a a a − a −