1/12+1/20+1/30+1/42+1/56+1/72=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)=1/3(-1/4+1/4)+(-1/5+1/5)+(-1/6+1/6)+(-1/7+1/7)+(-1/8+1/8-1/9)=1/3-1/9=2/9