f(x)=
x 2+
x,数列{a n}的前n项和为S n,点(n,S n)(n∈N *)均在函数y=f(x)的图象上.
(Ⅰ)求数列{a n}的通项公式a n;
(Ⅱ)令b n=
,求数列{b n}的前n项和T n
(Ⅰ)∵点(n,S n)在f(x)的图象上,
∴S n=
n 2+
n.
当n≥2时,a n=S n-S n -1=n+1;当n=1时,a 1=S 1=2,满足上式,∴a n=n+1(n∈N *).
(Ⅱ)b n=
=
,
T n=b 1+b 2+…+b n=2+
+…+
,①
T n=
+
+…+
+
,②
由①-②,得
T n=2+
+
+…+
-
=(1+
+
+…+
)+(1-
)=
+1-
=2(1-
)+1-
,
∴T n=6-
.
略