Question

2分之1+6分之1+12分之1+20分之1+30分之1+42分之1+56分之1+72分之1+90分之1+110分之1

Answer 1

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90

=（1-1/2）+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……+(1/9-1/10)

=1-1/10

=9/10