答:1/2+1/6+1/12+1/20+......+1/90 =1/2+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/9-1/10) =1/2+1/2-1/10 =1-1/10 =9/10
1/2+1/6+1/12+1/20+......+1/90=1/2+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/9-1/10)=1/2+1/2-1/10=1-1/10=9/10