按一定规律排列的一列数依次为:1⼀2,-1⼀3,1⼀10,-1⼀15,1⼀26,-1⼀35......,按此规律排列下去第九个?第n个?

2025-01-04 23:49:27
推荐回答(1个)
回答1:

an=(-1)^(n+1)/[n^2+(-1)^(n+1)]

a9=(-1)^(9+1)/[9^2+(-1)^(9+1)]
=(-1)^10/[81+(-1)^10]
=1/82