1/2X1/3-1/3X1/4-1/4X1/5-1/5X1/6.1/39X1/40

1/2X1/3-1/3X1/4-1/4X1/5-1/5X1/6.1/39X1/40
1/2X1/3-1/3X1/4-1/4X1/5-1/5X1/6.1/39X1/40

1/2X1/3-1/3X1/4-1/4X1/5-1/5X1/6.1/39X1/40
很高兴为您解答!
原式=1/2-1/3-1/3+1/4-1/4+1/5-1/5+1/6.1/39+1/40
=1/2-2/3+1/40
=-17/120

1/2X1/3=1/2--1/3
1/3X1/4=1/3--1/4
.............................
依次类推最后结果=1/2--1/140=69/140