1/2 = 1/1*2 = 1/1 -1/2
1/6 = 1/2*3 = 1/2 -1/3
1/12 = 1/3*4 = 1/3-1/4
所以前三十个的和
=1 -1/2+1/2-1/3+……1/30-1/31
=1-1/31
=30/31