原式=1+1/3+1/6+...+2/[n(n+1)]
=1+1/3+1/6+...+2[(1/n)-1/(n+1)]
=1+2[1/2-1/3+1/3-1/4+...+1/n-1/(n+1)]
=1+2[1/2-1/(n+1)]
=2-2/(n+1)
=2n/(n+1）
重点是了1/[n(n+1)]=1/n-1/(n+1)