这个用裂项求和1/(n²+3n+2)=1/(n+1)(n+2)=1/(n+1)-1/(n+2)Sn=1/2-1/3+1/3-1/4+1/4-1/5+........+1/n-1/(n+1)+1/(n+1)-1/(n+2) 相互抵消Sn=1/2-1/(n+2) =n/[2(n+2)]
