1. nx(n/n+1)= n-(n/n+1)2. 1x1/2=1-1/2,2x2/3=2-2/3,3x3/4=3-3/4，…… (1-1/2)/10x(2-2/3)/9x(3-3/4)/8x.....x(10-10/11)/1=(1x1/2)/10x(2x2/3)/9x(3x3/4)/8x.....x(10x10/11)/1=10/11
