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能說說第三步哪兒這么做的原因嗎?不能也沒關(guān)系的啦~~我只是怕老師問起來我講不出來~~1/n*(n+1)=1/n-1/(n+1)