问题
解答题
设Sn是正项数列{an}的前n项和,且Sn=
(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
+a 21
a1-1 2
,解出a1=3.3 4
(2)又4sn=an2+2an-3①,可得 4sn-1=
+2an-3(n≥2)②,a 2n-1
①-②4an=an2-
+2an-2an-1 ,即 a 2n-1
-a 2n
-2(an+an-1)=0,a 2n-1
∴(
+an-1)(an-an-1-2)=0,a n
∵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.