写出下列数列的通项公式:(1):1/2,-1/2,5/18,-7/54 (2):3,8,15,24,35
问题描述：
写出下列数列的通项公式:(1):1/2,-1/2,5/18,-7/54 (2):3,8,15,24,35
答
(1)
1/2,-3/6,5/18,-7/54
[(-1)^(n-1)]*(2n-1)/[2*3^(n-1)]
=(2n-1)/[2*(-3)^(n-1)]
(2)3,(3+5),(3+5+7),(3+5+7+9),(3+5+7+9+11)
3+5+7+9+...+(2n+1)=(3+2n+1)*n/2=n(n+2)
答
(1):[(-1)^(n+1)(2n-1)]/2(3^n-1)
(2);n^2+2n
答
1/2,-1/2,5/18,-7/54
1/2,-3/6,5/18,-7/54
an=(-1)^(n+1)*(2n-1)/[2*3^(n-1)]
an=(n+1)^2-1