1/12+1/20+1/30+1/42+1/56+1/72+1/90

=1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30
【通项：
1/n(n+1)=1/n-1/n+1】