1-1/2+1/3-1/4+...+1/19-1/20=(1-1/2+1/3-1/4+1/5-1/6+1/7-1/8+1/9-1/10)-(1/12+1/14+1/16+1/18+1/20)+(1/11+1/13+1/15+1/17+1/19)=1/11+(1/3-1/6-1/12)+1/13+(1/7-1/14)+1/15+(1-1/2-1/4-1/8-1/16)+1/17+(1/9-1/18)+1/19+(1/5-1/10-1/20)=1/11+1/12+...+1/20=155685007/232792560
1/11+1/12+...+1/20=155685007/232792560
155685007/232792560