a[n+1]/an=（n+2）/n是有个括号吧？a[n+1]/an=（n+2）/n=（n+2）*（n+1）/【（n+1）*n】那么移项：a[n+1]/【（n+2）*（n+1）】=an/【（n+1）*n】=……=a1/1*2=1/2【观察an/【（n+1）*n】，可以递推到a1/1*2=1/2，】an=n(n+1)/2
