等差数列{an}的前n项和为Sn,且S4=40,Sn=210,Sn-4=130,则n=__S4=a1+a2+a3+a4=40 Sn-Sn-4=an+an-1+an-2+an-3=80∴4(a1+an)=120.∴a1+an=30.∴Sn=((a1+an)×n )/2 =(30×n )/2 =210.∴n=14.问:Sn-Sn-4=an+an-1+an-2+an-3=80 这一步怎么
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