裂项法: 1/6+1/18+1/36+1/60+1/90 =1/3×(1/2+1/6+1/12+1/20+1/30) =1/3×(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6) =1/3×(1-1/6) =1/3×5/6 =5/18