1／2 + 1／6 + 1／12 + 1／20 + 1／30+ 1／42 + 1／56 + 1／72
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5) +(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
=1-1/9
=8/9
（1+ 1／2+ 1／3+ 1／4+ 1／5）×（1／2+ 1／3+ 1／4+ 1／5+ 1／6）-（1+ 1／2+ 1／3+ 1／4+1／5+ 1／6）×（1／2+ 1／3+ 1／4+ 1／5）
=(1+1/2+1/3+1/4+1/5)×[(1/2+1/3+1/4+1/5)+1/6]-[(1+1/2+1/3+1/4+1/5)+1/6]×(1/2+1/3+1/4+1/5)
=(1+1/2+1/3+1/4+1/5)×(1/2+1/3+1/4+1/5)+(1+1/2+1/3+1/4+1/5)×1/6-(1+1/2+1/3+1/4+1/5)×(1/2+1/3+1/4+1/5)-(1/2+1/3+1/4+1/5)×1/6
=(1+1/2+1/3+1/4+1/5)×1/6-(1/2+1/3+1/4+1/5)×1/6
=(1+1/2+1/3+1/4+1/5-1/2-1/3-1/4-1/5)×1/6
=1×1/6
=1/6