给学生自主发挥的空间 a n+11+2a 1 am+2 三 十 n+1 2 a=b(b>0,a.≤ma11,n=2,34, n+an-1 n n n a a a 2 1 2 1 2 1 = + + + n n n a a a 1 2 1 + = + ( 0), , 2,3,4, 1 1 1 = + =   − − n n a na a b b a n n n 给学生自主发挥的空间