imxd
幼苗
共回答了18个问题采纳率:94.4% 举报
2/12 + 2/20 + 2/30 + 2/42 + 2/ 56 +2 /72 + 2 /90
=(1/2- 1/3)+(1/2 -2/5) + (2/5- 1/3) +(1/3- 2/7)+(2/7 - 1/4)+(1/4 - 2/9)+(2/9 - 1/5)
=1/2 - 1/3 + 1/2 - 2/5 + 2/5 - 1/3 +1/3 -2/7 + 2/7 - 1/4 +1/4 - 2/9 + 2/9 - 1/5
=1 -1/3- 1/5
=7/15
1年前
6