=2/3+1/12+1/20+1/30+1/42=3/4+1/20+1/30+1/42=.......=6/7
1/2+1/6+1/12+1/20+1/30+1/42=1/(1*2)+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)+1/(6*7)=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7=1-1/7=6/7
是学生的奥数题^_^
