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{bn}的前n项和

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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{bn}的前n项和设等差数{an}的前n项和为Sn已知a3=5,S3=9(1)求

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{bn}的前n项和
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{bn}的前n项和

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{bn}的前n项和
s3=a1+a2+a3
s3=3a2
3a2=9
a2=3
d=a3-a2=5-3=2
a2=a1+d
a1+2=3
a1=1
an=a1+(n-1)d
=1+2(n-1)
=2n-1
a2n=2*(2n)-1
=4n-1
bn=a2n
=4n-1
sbn=4*1-1+4*2-1+.+4n-1
=4*(1+2+.+n)-n
=2n(n+1)-n
=2n^2+2n-n
=2n^2+n

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