设数列{An}的前n项和Sn=2a-2n平方.1求a3,a4

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设数列{An}的前n项和Sn=2a-2n平方.1求a3,a4

设数列{An}的前n项和Sn=2a-2n平方.1求a3,a4
设数列{An}的前n项和Sn=2a-2n平方.1求a3,a4

设数列{An}的前n项和Sn=2a-2n平方.1求a3,a4
an=Sn-S(n-1)=2a-2n^2-[2a-2(n-1)^2]=-4n+2
即当n>1时,an=-4n+2
那么
a3=(-4)*3+2=-10,
a4=(-4)*4+2=-14

an=Sn-S(n-1)=-4n+2
a3=-10
a4=-14

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