原式=1/3+1/4-(1/4+1/5)-(1/5+1/6)-(1/6+1/7)-(1/7+1/8）
=1/3+1/4-1/4-1/5-1/5-1/6-1/6-1/7-1/7-1/8
=1/3-1/8
=5/24