Question

已知{an}是首项为19,公差为-2的等差数列,Sn为{An}的前n项和,(1)求通项a、b及前n项和S

Answer 1

1.通项:an=19+(n-1)*(-2)=21-2n

Sn=(a1+an)n/2=(19+21-2n)n/2=-n²+20n

2.bn-an=3^(n-1)

bn=21-2n+3^(n-1)

{bn-an}的前n项和为(1-3^n)/(1-3)=(3^n-1)/2

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