Question

设等差数{an}的前n项和为Sn已知a3=5,S3=9(1)求{an}的首项a1和公差d的值(2)若bn=a2n,求数列

Answer 1

（1）S3=3a2=9

a2=3

则d=a3-a2=5-3=2

a1=1

(2)bn=a2n=1+2*(2n-1)=4n-1

Sn=(3+4n-1)*n/2=2n^2+n