1/12+1/20+1/30+……+1/90=(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+……+(1/9-1/10)=1/3-1/4+1/4-1/5+1/5-1/6+……+1/9-1/10=1/3-1/10=10/30-3/30=7/30
