问题 解答题
设Sn是正项数列{an}的前n项和,且Sn=
1
4
an2+
1
2
an-
3
4

(1)求a1的值;
(2)求数列{an}的通项公式;
(3)已知bn=2n,求Tn=a1b1+a2b2+…+anbn的值.
答案

(1)当n=1时,由条件可得 a1=s1=

1
4
a21
+
1
2
a1-
3
4
,解出a1=3.

(2)又4sn=an2+2an-3①,可得 4sn-1=

a2n-1
+2an-3(n≥2)②,

①-②4an=an2-

a2n-1
+2an-2an-1 ,即
a2n
-
a2n-1
-2(an+an-1)=0

(

an
+an-1)(an-an-1-2)=0,

∵an+an-1>0,∴an-an-1=2(n≥2),

∴数列{an}是以3为首项,2为公差之等差数列,

∴an=3+2(n-1)=2n+1.

(3)由bn=2n,可得Tn=3×21+5×22+…+(2n+1)•2n+0③,

2Tn=0+3×22+…+(2n-1)•2n+(2n+1)2n+1④,

④-③可得 Tn=-3×21-2(22+23+…+2n)+(2n+1)2n+1=(2n-1)2n+1+2,

Tn=(2n-1)•2n+1+2

单项选择题
单项选择题