Question

1/3,-5/7,3/7,-4/9.第7个数是什么,第N个数是什么?

Answer 1

∵1 3 =1 2×1+1 ,

-2 5 =（-1）1 2×2+1 ,

3 7 =3 2×3+1 ,

-4 9 =（-1）4 2×4+1 …,

∴第七个数为7 2×7+1 =7 15 ,

∴第n个数为(-1)n+1n 2n+1 ．

故答案为7 15 ,(-1)n+1n 2n+1 ．

顺便说一下这是2011贵州六盘水的18题