1/6+1/12+1/20+1/30+.1/72+1/90
=1/2*3+1/3*4+1/4*5...+1/9*10
=1/2-1/3+1/3-1/4+1/4...-1/10
=1/2-1/10
=2/5 应用公式1/n(n+1)=1/n-1/(n+1) 不理解可以问