等差数列{an}中,Sn=18,an+an-1+an-2=3,S3=1n=?_数学解答

等差数列{an}中,Sn=18,an+an-1+an-2=3,S3=1
n=?

人气：224 ℃　时间：2020-01-25 12:55:40

解答

设an=a+(n-1)dSn=[a+a+(n-1)d]xn/2=[2a+(n-1)d]xn/2=18an+an-1+an-2=a+(n-1)d+a+(n-2)d+a+(n-3)d=3a+(3n-6)d=3S3=a+a+d+a+2d=3a+3d=1由3a+(3n-6)d=3和3a+3d=1解得a=(n-5)/(3n-9) d=2/(3n-9)将a、d代入[2a+(n-1)d]xn/...