(06山东) n n n a 1=a,(an+2)→1=1 111 a a (a+2)2 a a+2 2 2 a a a +1 +l• （06山东） = + = + = + = + n k n k n n n n n n S b a a a a a b 1 2 1 , 2 1 1 2 , 求 ) 2 1 1 ( 21 ( 2) 1 1 ( 2) 1 1 + = − + = +  = + + n n n n n n n n a a a a a a a a 1 1 2 2 ) 1 2 ( 1 + + = + − = − n n n n n n a a a a a b