n>=2时,an=Sn-Sn-1=n^2an-(n-1)^2an-1(n-1)^2an-1=(n^2-1)an因为n-1不等于0,所以(n-1)an-1=(n+1)anan/an-1=n-1/n+1累乘：an=an/an-1*an-1/an-2*an-2/an-3.a3/a2*a2/a1*a1=1/n(n+1)当n=1时,a1=1/2,满足an=1/n(n+1)综...为什么累乘完an会等于1/n（n+1)?那累乘这懂了么