求1+1/1+2+1/1+2+3+1/1+2+3+4+…+1/1+2+3+…+n,(n∈N*).
问题描述:
求1+
+1 1+2
+1 1+2+3
+…+1 1+2+3+4
,(n∈N*). 1 1+2+3+…+n
答
∵ak=
=1 1+2++k
,2 k(k+1)
∴Sn=2[
+1 1•2
++1 2•3
]1 n(n+1)
=2[(1−
)+(1 2
−1 2
)++(1 3
−1 n
)=2(1−1 n+1
)=1 n+1
.2n n+1