1又1/2+3又1/6+5又1/12+7又1/20+9又1/30+11又1/42
=(1+3+5+7+9+11)+(1/2+1/6+1/12+1/20+1/30+1/42)
=(1+11)×6/2+(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7)
=36+1-1/7
=36+6/7
=36又6/7能说说第三步哪儿这么做的原因吗?不能也没关系的啦~~我只是怕老师问起来我讲不出来~~1/n*(n+1)=1/n-1/(n+1)