lim(-5)^n+(3)^n+2/(-5)^n+1+3^nan=1/n(n+1)1≤n≤33×1/2^n-1n≥4求an的极限sn的极限

Question

lim(-5)^n+(3)^n+2/(-5)^n+1+3^n
an=1/n(n+1) 1≤n≤3 3×1/2^n-1 n≥4 求an的极限 sn的极限

Answer 1

an=1/n(n+1) 1≤n≤3 3×1/2^n-1 n≥4 求an的极限 sn的极限,
这里好像没有阐述明白.
最好说的详细些.