1/8加1/24加1/48加1/80加1/120
=1/4×(1/2+1/6+1/12+1/20+1/30)
=1/4×(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6)
=1/4×(1-1/6)
=1/4×5/6
=5/24