(1/2)+(1/6)+(1/12)+(1/20)+(1/30)+.+(1/42)
=1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7请看清楚后面是什么！(1/2)+(1/6)+(1/12)+(1/20)+(1/30)+......+(1/420)=1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+......+1/20-1/21=1-1/21=20/21