原式=(1-1/8)+(1-1/24)+(1-1/48)+(1-1/80)+(1-1/120)=5-(1/8+1/24+1/48+1/80+1/120)=5-(1/2)x(1/2-1/4+1/4-1/6+1/6-1/8+1/8-1/10+1/10-1/12) =5-(1/2)x(1/2-1/12)=5-(1/2)x(5/12)=5-5/24=115/24