设bn={1,-1,1,-1...}=(-1)^(n+1)
设cn={1,2,3,4,...}=n
设dn={2,3,4,5,...}=n+1
则an=bn×(cn/dn)
=[(-1)^(n+1)]×[n/(n+1)]
=[n(-1)^(n+1)]/(n+1)