1／8+1／15+1／24+1／35+···+1／80+1／99=1/2*4+1/3*5+1/4*6+1/5*7+...+1/8*10+1/9*11=1/2(1/2-1/4+1/3-1/5+1/4-1/6+1/5-1/7+...+1/8-1/10+1/9-1/11)=1/2(1/2+1/3-1/10-1/11)=1/2x(11/15-1/11)=1/2x106/165=53/16...