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
