1/6+1/12+1/20+1/30+1/42+1/56=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8=1/2-1/8=4/8-1/8=3/8
五十六分之二十一
