据以上规律可得:1/4*1/5=1/4-1/5 1/5*1/6=1/5-1/6
1/2+1/6+1/12+1/20+1/30=1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=1-1/6=5/6