第一题1/(1+2+3+----+n)=1/[n(n+1)/2]=2[1/n-1/(n+1)]1/(1+2)+1/(1+2+3)+1/(1+2+3+4)+……+1/(1+2+3+4+……+50)=2(1/2-1/3+1/3-1/4+1/4-1/5+-----+1/50-1/51)(消去中间部分)=2(1/2-1/51)=49/51