由题意an=2sn ,a[n+1]=2s[n+1]
2(s[n+1]-s[n])=a[n+1]-an
2a[n+1]=a[n+1]-an
a[n+1]+an=0
原数列为1,-1,1,-1.
an=(-1)^(n+1)
注:[]为下标